I think Doyle gave interviews where he talked about disliking O'Hare (without referring to him by name), and in JMS's autobiography, he describes their dynamic this way (page 329):
Since [O'Hare's] symptoms were exacerbated by stress, I adjusted the remaining scripts to move other characters into the foreground, shaving off some of Michael's hours and reducing the weight he had to carry as the lead. This had an immediate benefit, and Michael began to exhibit fewer symptoms. His situation would've been almost managable had there not been a personality conflict with fellow actor Jerry Doyle, who'd figured out that there was something wrong with Michael. Despite not knowing the full extent of the problem, Jerry took every opprotunity to wind him up like a cheap watch just to watch him spin out.
Considering his final career as AM radio talk-jock, and some of Garibaldi's most brutal views being direct quotes of the actor (the "electric bleachers" crack, for instance), I'm not really surprised that Doyle had a taste for bullying a more "sensitive" man.