Originally Posted by
Howste
I'm not justifying what the other passenger did, but I generally prefer to put my bag on the side opposite from my seat because then I can see if somebody tries to mess with it.
Same here. And in my experience if you are carrying something expensive or sensitive for your employer they instruct you to put it on the opposite side bin if it won't fit under your feet for just that reason.
To the OPs question though - You snooze you lose if you were sitting down and fiddling with your bag but if you were standing up and just grabbing a few things out then it is a dick move to for somebody to toss a bag in to a spot that someone else is obviously about to put their bag in to it.