What affects them is load and the condition of the grid that they are pumping electrons into.
The recent revelation of the cause being pin-pointed to the loss of three lines in Ohio have me scratching my head.
Tornadoes, straight line winds and lightning take these lines our all the time.
Line repairs, etc. require de-energizing of large transmission lines in portions of a grid all the time. Switches at substations fail, transformers catch fire and numerous faults occur on the grid without dropping vast portions of a grid.
As to power reversals, A/C power flows in both directions. A reversal only indicates that the Ohio area or somewhere up line lost generating capacity and the NY generators began to fill the gap causing them to disconnect due to the load.
If it were the case that the downed lines caused this, then why did numerous switches not dis-engage that portion of the grid before New York got popped with the entire load?
Bad engineering on their loop is one possibility. Another is that the reason stated is not the cause, but an effect.(as I said, I am quite sure they have had failures on these lines before without serious consequences)
All and all, NY and others only suffered a short outage. Most folks have had many of these before, but some how NY is special or something. It boggles my mind.(I have often had to read my electric meter with a flashlight. We even have a special place on the wall in the living room to make shadow figures when we go to candlelight)