IIRC there are 2 equal and one smaller cubby spaces on the 738 and the aisle has the smaller space. In this case the windows passenger was claiming both of the larger under seat spaces. Sorry, IMO the window seat passenger was out of line here and deserved the attitude from the aisle passenger.
When there are 2 larger and 1 smaller spaces, the two larger spaces should be shared between the two seatmates. That's the equitable thing to do. Shame on AA for not making the smaller space always end up in the middle but that's the way it is. And I say this as someone who typically sits in the window seat and willingly gives up the middle space in order to be fair.