Keyshawn Johnson -- I Went All HULK Outside Justin Bieber's House ... He's A 'WEASEL'

Keyshawn Johnson says Justin Bieber acted like a weasel after allegedly roaring through his Calabasas neighborhood and then scampering into the house ... but Keyshawn took matters into his own hands ... prying Justin's garage door open and then slamming a bodyguard against a wall.

Keyshawn recounted the story recently to a packed classroom at USC -- Keyshawn's alma mater.

TMZ broke the story ... Keyshawn chased Bieber's Ferrari down last May ... IN HIS PRIUS ... to lecture him about the dangers of speeding in a residential area.

But Keyshawn's version of the story is WAY more insane -- you have to hear it for yourself.