I work in a lovely startup. People are very nice, we have a lot of Directors and VPs but communication is good and nobody usually complains about management. We recently started hiring heavyweights from some other companies. They started reorganising the company, reassigning middle managers and hiring former colleagues of theirs in more senior roles. More importantly, communication with some of these managers is much different: they make it clear that "I am a MANAGER, and who the hell are YOU?". While I was talking to all levels, with these people now I hear "you should ask your line manager for this", i.e. they won't talk to me but to my manager. I can understand this, but I wonder: as these guys come from bigger companies, is this what I should expect from management in general? Am I expected to have a negative experience or at least to be seen as a foreigner, unless I am part of the "old boys club"?
You should expect things in a startup to change rapidly and often. That is part of the nature of startups.
Roles change. People that were formerly the go-to folks technically find themselves further down the totem pole and no longer relied on as much. New people with new management styles are brought in. Some stick, others move on quickly. The one constant is change. So it goes in many startups.
When you go through the phase of rapid hiring of middle-level managers, things are often chaotic. Many of these managers come from very different environments, and bring a very different management style. A good startup has a strong culture and knows how to convey this culture and expectations to their new hires effectively.
But sometimes, it takes time. And sometimes it isn't as effective as it should be.
I think you already know that management which "treats people like crap" isn't to be expected. For me, it wouldn't be tolerated. If I hired those middle managers and they didn't change, they would be fired. And if I worked for such a middle manager and the company tolerated it for longer than a few months, I would leave.
Give it a little while and see if things settle out for the better. If not, act accordingly.