Apologies if it's been mentioned already, but perhaps this pterosaur was being a nuisance. If a spinosaurid had landed a very large fish, or was working on a carcass, the pterosaur might have taken advantage of the opportunity to scavenge a bit of flesh, and been dealt a fatal bite. That might explain why it wasn't devoured - a larger meal was already present. _Irritator_ became irritated, and flung the squawking #$%&* across the river, probably followed by a satisfied snort.
(scratching chin thoughtfully)