A Boggart had made its home there, and as soon as Sirius reverted to human form, it began trying to drive him away.
"Sirius!" James snapped. "How could you be so stupid? You should have known what Peter was. We chose him as the Secret-Keeper because of you. It's your fault Lily and I are dead."
The form changed. Remus spat only a single word.
Lacking a wand, Sirius could only become Padfoot. As the Boggart advanced, he backed away, whimpering, to find other shelter.