The reason he grabbed the door handle is because in his mind he thought he was going to be the one knocking the officer down. He's a big and very aggressive guy that's this spry at 79 - I'm positive this wasn't even remotely close to his first rodeo. He grabbed on to help maintain balance.
As another issue he wasn't running anywhere in particular, except towards the officer. As soon as he collides, he then gets up from crashing into him he turns around and starts racing back towards him again. He then pushes the other officer at 28 seconds and begins grappling with yet a third officer at 30 seconds. He's then tackled at 35 seconds.
Given he was not arrested for intentionally crashing into the first officer I think ICE was generally trying their hardest to ignore him, but that probably became impossible about the point he actively decided to start grappling with them.