I think Donnie knows about Raph's or Leo's crush on the other because in "The weird world of wyrm" when the turtles are running from Casey, Donnie immediately drops Leo once raph comes into the panel.
~OR~
Donnie has a crush on Leo, but knows Raph does too, so he tries to impress his crush by helping him, and when raph comes into the panel, drops him either cuz he doesn't want raph to get suspicious, or get angry with him later
IDK I kinda think the first one is more believable XD
