I still haven't heard a plausible theory related to my original question of 'what was this guy doing'? He obviously wasn't rescuing me since it started right at the beginning of the dive...he wasn't concerned about my health status or he would have asked a question or two after we came up...he wasn't preventing me from hitting the bottom, since I wasn't doing that...he wasn't teaching me anything...and I really don't think he was just doing it to be an *******, because I think that would have come along with some other rudeness up on the surface. That's the part I still wish I could understand.
A few people have talked about "trust me" dives. Like
@The Chairman points out, a lot of us did "trust me" dives when we first started out. Mine was dive #6, not including training dives. In addition to learning to call a dive or having the confidence to just end the dive on your own, I'd say learn to get people talking.
What I do and why I do it is often related to past experience. This Divemaster did what he did based on past experience. It could be past Divemaster experience, past scuba diver experience, past childhood experience. It is impossible to know now why he did what he did, unless you get him talking.
However, one of the reasons I chat up the people going on a dive, even before I was a Divemaster, was to understand what baggage people were bringing to the dive. If I was driving, as a customer, to a cenote with this guy then I would have tried to chat him up. Maybe if you could get him to loosen up and start talking to you, you might have learned something about him and how to 'handle' him. Or maybe he might have learned a bit about you and dropped some of his baggage. But the reality might have been he never opened up to you. In this case, I'd tell him straight up, his silence and unwillingness to talk to me makes me not trust him. I'm not going for the dive. I'd also bring it up to the shop. I'm not going to worry about if he hates me. Whether or not he hates me doesn't matter when you consider I'm trying to make myself safe from harm or death.
By the way, the second cenote dive I went on someone said something about how would they know how to end the dive, i.e. how do they take care of themselves because they don't want this to be a "trust me" dive. The guide told them how they could just cover their light, do a 360 degree and at some point they'd see light. Swim for the light and you'll be able to exit the dive on your own. The guide was actually happy the diver wanted to be able to take care of themselves. If he wasn't, I'd REALLY not want to dive with him.
P.S. The fact you are posting this here means you are talking and learning. Keep it up. Learn to do it in person too.