To me it seems like the guy on the Yamaha was looking for trouble. There were plenty of other gates open to the inside. It's not like it was a full gate. Then lol at 0:51 when he yells at his mom to get the owner.
I'm sure there's more to this then what's in the video, but since I don't know that I'm just going by what I see.
So I did look in the AMA Rulebook and this is the only thing I could find regarding starts.
The first applies more to the track though.
SECTION 1.3
C. Race Rules
9. Starts
b. At the starting line, the course must allow 1-meter
(3.2-feet) of starting space for each machine.
SECTION 1.3
C. Race Rules
9. Starts
d. When a rider chooses their location on the starting
line, they may not change that position.
So after you pick your gate you're kind of stuck unless the race official moves you. BTW I believe he was asking for the race official (who might also be the owner

) But yeah the whole start was a little childish, from both parties.
SECTION 1.3
C. Race Rules
13. A competitor who rides in a way that endangers officials,
other riders or the public will be subject to immediate
disqualification from the meet by the referee.
The Yamaha guy went down twice, once at the start and again after he passed the Suzuki guy. Did he take him out both times?

If he did then I defiantly believe the official made the right call by DQing him.