Oh, you're right. Why did I think it was the tiger that did it? Akane reacted to the tiger more than Ranma did; he barely seemed to notice it except when it roared, and even then it was just his hair standing on end and that could be just from the volume.*
And looking at it, doesn't it seem that it's the combination of cats and no way out that really breaks his control? He's backed into the corner by a tiger, a corner filled with cats just a scared as he is. He seemed to be holding on, kind of, until that point.
*I really need to go back and reread the series. I haven't done a straight reread in almost 10 years.
Come to think of it, a lot of my stuff is closer to two decades old rather than one.