I have had a Captain hitting the autobrake pb on landing too. I was relatively new and we were landing in medium, as per company policy. I'd literally just touched down when I noticed him lean over slightly, followed by the brakes "failing". Obviously I continued with manual braking, but he said nothing. Once at a safe speed I asked him about it, and he just said "well we had lots of runway left so I thought we didn't need medium autobrake". I suggested that he tell me if he wanted me to go manual braking, and if he did decide to hit a button, he should have mentioned it anyway.
However, this is the same Captain, who, on an Airbus, decided to "help" once with a landing of mine. He had decided that it was possible I might not flare enough (where he got that from I have no idea, I hadn't done a bad landing up until then). As I had already started the flare, his input (without telling me) meant that we did a bit of a balloon. With 2 of us putting inputs to the stick, we got it on the ground.
On the taxi in he admitted to "helping me with the flare". I mentioned that was why we had ballooned, and why had't he called "I have control" or pushed the takeover pb? He didn't do that again, and I hope he never will.
None of this was standard procedure, obviously, but it is a bit worrying in the right hand seat, especially on a new type, when the Captain excludes you from the loop and insists on doing non standard things without even telling you.