Companions can push you and they do it a lot. It's a little frustrating when i'm being stealthy and Ellie might come over to me and push me off of cover. It happens a lot actually. I don't know why but it's only in cover when they can push you but anywhere else they just go right through you. It's only a small problem but it doesn't ruin the game.
I've never had it happen. I can sit behind cover for days with Ellie spazzing about, sometimes deciding to crouch next to me and get up in my shit, and Joel never gets pushed and inch. I've had times where Ellie tries and fails to run through me, and after a few seconds she just clips through me while I remain still, the exact same way it happens if you trap her in a corner or in a narrow hallway and keep walking at her.