Detector rails are being powered by a minecart passing by it on a nearby, but unconnected, track. Specifically, it seems to only happen when the minecart has just turned onto a powered rail that is directly next to (but not connected to) the detector rail. I tested it a lot, and I found that whether or not the bug happens seems to depend on several variables:

• If the turn and the powered rail are in different chunks, the bug doesn't happen (see video 1)
• If the powered rail and the detector rail are in different chunks, the bug doesn't happen
• The y coordinate and the direction the minecart is traveling in don't seem to have any effects
• The powered rail can be replaced with an activator rail or another detector rail without changing the result
• The minecart's type and/or speed affects the result somehow
• The path the minecart follows before reaching the turn affects the result somehow
• Even tiny changes in the minecart's position before it starts moving can change the result (see video 2)
• No setup that I tried caused the bug to only sometimes happen

Most of the testing that I did used the hopper minecart, in case other minecarts don't behave similarly for some of these.