It’s well known the reason Amtrak sucks is because freight trains run on the same tracks and their sidings are too short, relative to the length of the trains, to prevent freight-caused delays on most Amtrak lines. Avg. passenger rail speeds in the US peaked in the 1880’s iirc.
If you simply can’t use a siding because your train is too long, I can see a derailment becoming more likely.
There may also be loading fatigue contributions, similar to higher speeds and harder tires making semi trucks responsible for 90+% of US highway wear. The longest trains no doubt run on a minority of busiest routes, so there is less risk of derailment with shorter trains on the plurality of routes. In essence, it’s not that the derailed train is too long, it’s that the previous train is too long, causing the track to wear out too fast.