I guess that last scene offers 2 options:

-Eren needs some time to control the titan he creates. During that period, he attacks anything. Or he's so exhausted he can't control his Titan. Or, when not other threat is around he falls back to humans. Great power need control and experience, he obviously lacks in those since he just discovered his powers.

-The other option is the usual: it seems like I'm attacking you, but in fact I'm saving you from a threat behind/below/around you. Like there was a titan hidden below Mikasa. Overused, but why not.