From the novelization: "For all K-2’ s social dysfunction (or perhaps his disinterest in organic socialization— who could fathom the mind of a droid?), he knew Cassian better than anyone. He’d seen Cassian commit acts even Draven wasn’t aware of.

On Jenoport, he’d found Cassian staring at his blaster with tears on his face. K-2 had volunteered for a memory wipe in case Cassian’s 'continued dignity and service demanded it.'"

Somebody tell me the sad story of what happened on Jenoport?