I think about this guy and also Admiral "There Are So Many Uncharted Settlements" Ozzelfrom Empire sometimes.
My pet theory is that, from their perspective, Vader is primarily a gigantic pain in the ass. He has no official rank; he's just the Emperor's buddy. But he's constantly commandeering Imperial resources, up to and including entire fleets, diverting them away from carefully coordinated military planning, because he has "a feeling" that some survivor of Order 66 is hiding out on some backwater.
So then the entire damn fleet needs to go on a side quest, possibly destabilizing some other part of the galaxy where they're supposed to be part of a blockade ordered by the Admiralty, and Vader goes down with a platoon of expensive troops and equipment, and maybe even a bunch of them get killed or lost, and Vader flies back up to the Star Destroyer and announces "mission accomplished," because he managed to kill one guy that no one has even heard of.
So now the fleet is out of position, the Admiral is probably all pissed off because his orders got overridden, and you better believe that neither Vader nor the Emperor is going to hand out any medals for any of this. And God forbid Vader doesn't find the guy he's looking for! If that happens, he's even more pissed off than usual, and liable to cause even more property and personnel damage when he gets back to the ship. You're trying to run a fleet and subjugate a galaxy, and the Emperor and his Best Buddy with no discernible military experience whatsoever are constantly screwing it up with their weird personal vendettas.
So it's no wonder that the serious-minded military types are totally fed up with them, and maybe this guy figured that now that they can blow up planets they won't have to tolerate any more of Vader's bullshit.