"This guy was a meathead with no talent. And also he was a bully as a kid too!"
I mean, yeah, maybe this is true or partly true. But "Mannfred probably throwing shade and some misdirection too, and being a prick" still checks out as a viable partial explanation.
On the one hand, maybe there was something worthily punchable about Ushoran's face, since all the other bloodlines decided to dunk on him collectively. On the other hand, maybe they decided to dunk on him because they saw him as a threat to them (his kingdom was a threat to their dominance, or to their preferred modus operandi of working through the shadows; if he could set up a kingdom of vampires, and if he could reach out to other nations as well, then how much damage could he do to Neferata's get? would he be able to arm the mortal nations of knowledge of how to spot and eliminate Lahmians and other vampires? etc etc -- or perhaps for other reasons. Or perhaps he just got a stable kingdom up faster than they did, and they didn't have as much power or stability or security or dominance yet, but they did have spying operations and softpower or intrigue stuff and so could use that to topple him.) and then talked smack about him afterwards. Or maybe it was old grudges and vengeance from Nehekharan days or from the Nagash wars. Or maybe he really was getting together and preparing to war against the Lahmians or all other vampires as a whole or something and they decided to put a stop to that.