Power transmission and distribution lines are very overloaded in the summer. A rule of thumb for distribution transformers is 200% loading for  4 to 6 hours a day. When everyone goes to bed and the load drops the transformer can cool off. This is fine in average temps, but when it is in the 100's for weeks at a time it really stresses the system, things don't cool all the way down. They start into the peak load a little warmer than the day before. It gets worse each day. Heat and resistance are tied in a circle, heat increases resistance which increases heat, round and round it goes until a transformer fails. If it is a large substation transformer, the load it was carrying is shifted to the rest of the system. The problem is, there is no reserve capacity. The rest of the system is already overloaded. Trying to restore power after a large outage is a nightmare as well. Hundreds of manual switches must be opened. A feeder circuit is energized and load is added gradually and monitored. The process is halted if load is too great and time is needed to allow the load to drop before adding more load. It is a balancing act. A house of cards if you will. One day if we are not careful, we could loose most of the grid starting with something simple like removing a piece of monitoring equipment.

(note, I edited this post and added the paragraph above)

You have to listen between the lines. A worker removed a piece of monitoring equipment. I makes it sound like this worker woke up that morning and said to him or herself, "I think I will take the __________ alarm off the 154-19 transmission bus tie today" That is not how it works. Workers are directed, mainly by engineers. I would guess this piece of monitoring equipment was telling the engineers and the ISO (Independent System Operator an entity that ties transmission lines together for separate companies.) something was wrong. They could not figure out what was wrong and therefor called for the removal of the monitoring equip. I have no idea what type of monitor was involved but I have seen some pretty goofy stuff in 25 years in the trade. We had a panel in one of our 345KV bulk substations with a hand written note on it that stated: Do not bump or brush into panel, #12345 OCB (oil circuit breaker, big as a house) will operate. It would open the breaker and drop half of St Louis Metro area. This sign was on this panel for over three years. They finally replaced the panel last year after I texted pictures of it to one of the VP's

Dave

Here is a pretty neat video of some fellows working to keep the lights on. Very well done, I like how they filtered out most of the helicopter noise.

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