User blog comment:StarlightAT/Episode 6 Reactions/@comment-13616152-20130823181022/@comment-10390252-20130823181726

Because she previously didn't know that she can cut a tree down with one blow. She's so upset that she might not have realised that she just did so.

Discovering this power and learning to control it (let alone bring it out at will) will be part of Ruby's journey but I doubt it will be something that happens soon - for a long time, she's going to be mostly reliant on the tricks she built into Crescent Rose.