Partially just coincidence. I mean, she finds her ideal morph in book 7, and likes it so much she sticks with it. On the surface, at least, that's all there is to it.
But partially, I also think it's somewhat telling of her character. I mean, she went out looking for that bear because, well, the elephant was just not bad-a$$ enough. The only reason Cassie ever changed her morph at all was because horses cannot fight, and even wolves aren't that great for fighting, but it was good enough for Cassie, because, well, she's Cassie. Rachel was really the only character who, after fighting Yeerks for a while and 'getting into it' the way that she does, would decide "Hey, I think I need more firepower! I'm going to go find a bear."
Good question, by the way. Not many people think about things like that.