There were some things that were just instinct. James had been happily chatting away with his mates about something or other when a nasty explosion relatively close had nearly blown his ear drums out. He had ducked and covered instantly, wand in his hand before he even realized what he was doing. He thought he noted Peter transforming quickly and scurrying out of the way and when the second and third... were those bombs went off, James thought that was a bloody good idea.
There were loud popping sounds and... was that growling? Spells were going off in every direction, and James didn't have a clue what was going on, but he found himself running towards the danger, expecting Sirius was hot on his heels.
Death Eaters. He had no proof but he was still sure of it, and though the Aurors wouldn't be too far behind, they'd be far enough that the Death Eaters could cause some serious damage in the meantime. He was a skilled wizard. At the very least, he could give them something to shoot at.