The policeman ordered, right as he took a step out of his car "Turn it off!" - which is a lawful demand to prevent the biker from possibly kicking the gas and running. As the driver did not seem to comply (from the policeman's PoV) during his walk over to the bike, he enforced the order himself by turning off the bike and confiscating the key for the moment. Having made it safe that the driver couldn't leg it, he guided traffic around him so he could get to the side of the road.
We don't know what happened after the driver reached the curb to be lectured and/or arrested, the bike could be impounded or the confiscation might be temporary. So all we can do here is discuss the action of demanding that a motor vehicle be turned off, the doing of such and taking the keys. Demanding a vehicle to be shut off is standard procedure in police stop, as it is ensuring the safety of everybody involved.
In a somewhat recent case (trying to find it again!), a driver did not shut off the car and had to stand on the brake to keep it where it was. As commands came conflicting (keep your hands where we can see them, get out of the car!) and he could not comply or the car would jump forward and ram somewhere, things escalated and the driver was shot.
But back to the first step. Was the stop lawful? ACLU in NY tells us:
Police may stop and briefly detain you only if there is reasonable suspicion that you committed, are committing, or are about to commit a crime.
Don’t bad-mouth a police officer or run away, even if you believe what is happening is unreasonable. That could lead to your arrest.
There was a crime committed: Splitting is illegal in NY (among others: Section 1122, overtaking on the right), so the stop was justified under NY CPL 140.50. From my own experience, it is not uncommon for bikers to try to evade police by swerving back into traffic and using their higher mobility to get away. On its face, this makes it reasonable to demand the bike be shut off as the policeman advanced, and I'd like to congratulate the officer for taking the less escalating step and just turning the bike off himself on the noncompliance instead of drawing his gun and possibly escalating it to a use of force.
Most lawyers suggest to drivers pulled over to do things akin to "After you brought your car to a complete stop [on the curb], roll down your window and shut off the engine". Like this one.
Possibly confiscating the keys might be an overreach by the policeman, but the demand to turn it off clearly is not.