Ditto Strangles is to be taken very seriously, especially *******Strangles, and ditto that kind of behavioral change is something I've never heard of.
My Arab/Saddlebred was 27 when he went on his last organized ride --- and got a horrible horrible case of strangles.
It was found out later, the moron that let his horse drink straight from the big stock tank we were all supposed to bucket from, had the horse with strangles that spread it around. He should've kept the horse home and he REALLY should have had the snot knocked out of him for letting the horse drink from the big stock tank.
I was one of only a few that told him what an a** he was, and I wished I would've gotten in a smack-down with him because my horse and three others that I knew of, got sick.
The Strangles never traveled into Sonny's stomach but even the vet said he'd never seen a horse get that sick. I had to wear rubber gloves, bring him onto the road, break the sacks myself, thenn clean up what could into a plastic bag and throw clorox on the road. I was outdoor boarding, at the time, my horses were the only ones in the pasture but the vet did not want the lancing to be done in the pasture.
I had to lance the sacs as they were not wanting to break and the vet was worried the infection might travel to his stomach, where it could have been fatal.
The vet didn't see the need to come out and charge me for something he knew I could do, so he told me what to do over the phone and yes I was wearing rubber gloves.
I have changed a lot of diapers and I have cleaned up a lot of baby puke, but that was all a picnic compared to lancing those smelly-rotten-awful sacs under my horse's jowels and helping that stuff drain
While having Strangles never changed Sonny's "sunny" disposition, it did drag him down so much that he never fully recovered. I blame the strangles on exacerbating the cancer, which is why I had to lay him to rest two years later, when he was 29.
For the record, contrary to what I always believed, a horse does not get immunity from having Strangles. They can get it again but my understanding is, it won't be as severe
Unless something else happened at the same time as the Strangles, I honestly don't have an answer, except that Strangles shouldn't change a horse's behavior pattern like what you're describing