Realistically... Very easily? The default is to have auto-play on, I can imagine many scenarios where settings get lost. I've not heard general outcry about this so my best guess is that it's an edge case.
I've had it re-enable itself a few times. I'm sure it's due to the extra add-ons and such I use that's messing with it. A few of my friends don't use the same add-ons I use, or they don't use any and have had it happen enough times for them to complain to me about it.
My friend first found out his auto-play was re-enabled after youtube ate up most of his monthly bandwidth cap in one night.
So we're definitely in the minority, but it does seem to happen to different people with very different configurations.