I don't believe Mega Man is in any special state during up tilt. It just happens to be the only up tilt in the game that places the character in the air. However, the move lasts for just enough frames that Mega Man should not be capable of any other actions before landing on the ground. Players may have noticed you can go into a fall by up tilting on the end of a platform or ledge. Since you are airborne, an air dodge is possible once the frames of the move end, but that's all I can think of.
In addition, when performing this at the ledge, Mega Man will grab the ledge automatically. If you held down, you can make him not grab it and instead air dodge or do anything else.
My final wild theory was that this happened when you were fighting Game and Watch, and you whiffed an up tilt. While you were in the attack's animation, G&W used his Dtilt, which creates a wind box above the hitbox, causing you to remain airborne long enough for Mega Upper to end.