why: It looks like the droid was the last straw for Qui-Gon on a day when nothing went well.
Bit of a plot twist here:
Qui-Gon raises eyebrow "No?"
The unfortunate droid tries to escape but is cut down before making a move. Enter more droids and an angry Jedi master creating a general mayhem, just too happy to let out some steam. When the dust has settled and it is quiet again Qui-Gon switches off his saber, turns around and sees Obi-Wan.
"Feel any better, master?" he asks sarcastically.
/plot twist
Now, I don't know what'll happen after that, Obi-Wan gets his ass whupped?