The video of Ian Ziering brawling with a group of bikers in Hollywood tells the whole story -- at least according to one of the bikers who tells TMZ ... the whole incident was completely unprovoked by the motor crew.

We spoke to 2 men Wednesday on "TMZ Live" who sometimes ride with the 605 Minibike Gang, but are not members of the gang. They say Ian went from 0 to 100 all on his own, without any altercation prior to the video we've seen.

The 2 riders here go by Roy and Nocturnal. Nocturnal is the guy who was there on Sunday, and even recorded the video shot from behind Ian's SUV.

Nocturnal insists the group's first encounter with Ian was right there on Hollywood Blvd when the video starts.

He says 2 of his fellow minibikers were doing a legal lane split, and Ian purposely tried to prevent them from getting in front of his car ... even though they had every right to do so.

They say it appears Ian didn't like that ... and go on to allege he hopped out of his car to start a physical fight all on his own. If there was any incidental contact -- Nocturnal and Roy insist it was Ian's own doing ... and that he was the one who invited an ugly clash.

Nocturnal and Roy say they don't know why Ian might've escalated things the way he did -- especially with his young daughter right there with him -- but they theorize he was having a really bad day ... and let out his frustrations on the bikers.

As we first reported, LAPD is investigating the incident for felony vandalism -- for the bashing of Ian's windshield with a helmet -- and misdemeanor battery for the fight.

Ian, for his part, has suggested the bikers were at fault here ... and blamed cops for allowing "lawlessness" to take place. He hasn't responded to our calls to get his side of what set him off that day.