There were only so many things one could do when the earth started moving under and around you. In case of being an armed wizard, there were a few more options than there might normally be, so Sirius ended up throwing up a shielding charm and then exploded the wall lifting up in front of them with a blast.

In his defense, he really wasn't thinking about attacking Snape at that moment, just about the possibility of getting crushed. When he turned to face the woman, his wand was still out and feeling startled didn't really do much to make him less aggressive.

"Catching up."

He kept an eye on her, but did glance back at Snape, to make sure he'd not be leaving.