Crowley and Aziraphale had slept with each other. They had a discussion about it, how it could cause Aziraphale to fall, but they spent the night together anyway.

When they woke up, Crowley noticed first. Aziraphales eyes had gone from blue to red. His pupils serpentine. He had snake eyes too. And his wings had turned black.

Aziraphale choosing to love Crowley had caused him to turn into a fallen angel.