I suppose it could be from normal wear if you play a LOT.. But it doesn't seem likely since it looks like it has gone right through the ebony?  And I would think if it was wear it would be in more than one spot. 

Hard to say from a picture, but I would almost wonder if there was a problem with the wood there on the fingerboard like a deep gouge or ding and it got "fixed" with some sort of dark wood filling compound.

There is a "fix" that could be done on a rut like that, if you aren't going to be able to afford to get a luthier to put on a new fingerboard or tackle doing a fingerboard replacement yourself.  But I'll wait to let the more experienced folks chime in before talking about it, lest the rotten tomatoes start getting flung. LOL

Ok, I'll explain what I would do, short of buying a new fingerboard and cracking off the old one and gluing on a new one and then having to trim and plane it down and worrying about getting the 'scoop" and radius right and etc..

I have fixed similar ruts in guitar fretboards (I've never tried it on a violin, but logically it should work the same) with "super glue".  Cyanoacrylate.  You use the thin stuff, not the "gel" kind.  The gel tends to chip out easier, I don't know why.

First you clean the area with naphthalene.  That's lighter fluid, like "Zippo".  That will remove the finger oils, any traces of any polish, etc to get a clean surface for the glue.  If the fingerboard is dyed, it may also wipe off some of the dye, so it might be a good idea to check on an inconspicuous part of the fingerboard first to see if you're going to be wiping off some of the black.  Wet the damaged area down and wipe it out with a paper towel several times, to get it as clean is you reasonably can.

Once it is cleaned out, let it dry overnight to make sure there are no traces of the naphthalene.

Then you put in a few drops of super glue and spread them around with a toothpick.  Thin layer.  Don't try to fill it in just one or two coats.  Expect it to take several.  Let each layer dry about half an hour, and use a clean toothpick to touch a shiny part of it to see if it is actually still wet.  Don't use your finger to check it because you can feel very silly rummaging around for the nail polish remover to get loose from the glue with a musical instrument stuck to your hand.  As layers dry, keep adding a new layer until it is built up to just a teeny bit above the existing fingerboard.

Let it dry for at least a couple hours and then scrape or sand the "patch" down to feel level with the rest of the fingerboard.

That's it.  It does work, and the "patch" will usually hold up better than the original ebony did, since cyanoacrylate is harder than ebony.  It is also a very inexpensive repair.  That's the good points.

The main bad point is that it is clear and that can look a bit peculiar.  But no worse than it looks now, and playability should be just about exactly like it was when new.

So that's the basic trick/fix.  There are a couple of upgrades (mostly cosmetic) that you can do as well.

The first is if you can find a piece of scrap ebony or other dark wood to sand and get a bit of fine sawdust from.  If you sprinkle a little of that with each layer, you can get a better colour match.  The problem, of course, is that most people don't have scrap ebony or similarly dark hardwood just lying around.   

The second is a bit more work.  You can take something like a razor blade or X-acto knife and make shallow cuts along the grain of the wood in the damaged area.  you only want to go maybe a mm deep and make them parallel and close together, maybe half a mm apart of less.  Then after that bit of "shredding", go through with tweezers or something similar and lift the thin strips of wood you made, to break them loose from the main wood of the fingerboard just enough to get them to the top or a tiny bit above where your patch will come to.  Then when you fill in your layers of cyanoacrylate, it sets around the wood fibers you separated out and you preserve the look of the woodgrain more. 

If you do a good job of that, it is possible to fix a rut so that even on a very close inspection it would be hard to tell there ever was a rut there.  That's the good part.  The bad part is that it is a lot more work and time and I don't think it is any stronger than the simple patch I explained at the beginning.  Also doing a good job of it will take some luck to manage on your first try.

Some people may mention that you can add a little baking soda to the superglue to speed up the drying time.  While that is true, I don't do that trick in a fix like this.  If you add just a little too much, then you may not have enough working time before the stuff sets.  The baking soda also may not fully dissolve before the glue sets, giving you translucent white areas and you may get some air bubbles in it.  It's a good trick for some things, but not for filling a rut on a dark coloured piece of wood, in my opinion.