Malachi normally didn't go by the Divination shop Lavender worked at when she was at work or without a reason like the reading, but he got through with some tutoring that he hadn't ended all of early. Wanting to see her and knowing no one was expecting him home yet, he walked up Diagon Alley.
Before he got to the door of Bones in the Water however, he looked through the window. He saw Lavender and Mr. Pepper kissing. The sight stopped him dead in his tracks, just standing there for a moment. They were having an affair? Lavender and her boss? Hell, Lavender and anyone but him? They'd been through so much, given up so much to gain what they had... and she was just going to throw it aside for an affair with her boss?
Turning on his heel, he walked away. There was no way in hell he was going inside and let either of them - or both of them - lie to his face. Especially not Lavender. Once he was a far enough away, he Apparated home.