The way she struggled to say "grab on" suggests that, like Erin Titan, people in Titan form only freely act to achieve the specific goal they envisioned when transforming. Her transformation might have been "kill Titans and protect tower" when she transformed, making that the only thought she could elegantly process. It wasn't until mini blonde told her to bring down the tower that she began to improvise, and if Erin is any indication, it is hard enough to control your Titan form without processing external instructions after transforming. Those are good enough speculations for me to forgive Ymir's lack of tactics or running away.