Only your snake really knows why she is doing this. After her, you would be better able to discover the reason. If you want someone to offer their opinion of what they think might be going on, I will oblige. It's really long & just my best guess.
Short version: She's knocking to get out. You let her out, rewarding her for knocking. She repeats the behavior next time she wants out. You let her out again. Now she has trained you to let her out when she knocks. If you don't want her to knock, stop letting her out when she does it.
Long version: Her behavior doesn't sound strange to me. It sounds like she was looking for a way out. She started bumping against the glass & at some point you opened it, letting her out. If that is what she wanted, then you just rewarded her for knocking. To me, it sounds like your snake has done a good job or training you. "If I keep knocking on the door, he will let me out." We can't say why she started doing this but I'm going to guess she was looking for a way out. After that, you rewarded her for knocking by letting her out. Behaviors that get rewarded have a high likelihood of being repeated.
You can probably get her to stop by first making sure her cage is a place where she wants to be. After that you will need to teach her that knocking on the door will not get you to let her out. In theory, the solution is simple. Never let her out when she is knocking. Only let her out when she is NOT knocking.
Note that I said, "In theory..." The problem is that while we can train our pets to do things, we can also be trained ourselves.
If you don't like her behavior and can stop it by opening the door, then you have been rewarded for opening her door. That makes it more likely you will open the door the next time she knocks.
While it is best if you never open the door while she's knocking, you may not be able to always follow that rule. If for some reason you feel you must open the door while she is knocking, it would be best to open it as soon as possible when she starts knocking rather than waiting a long time & then finally giving in. If you wait a long time before opening it, then you are teaching her that persistence pays off. If you don't let her out early in the behavior then absolutely don't let her out at all. If she only has to knock a couple times before you open then she learns that she doesn't have to keep repeating the behavior. She also learns that if her first couple knocks don't get a reward, she isn't going to be rewarded at all. So you are basically shortening the duration of the time she will continue knocking. When you get to that point, you should be able to stop the behavior completely by ignoring it every time. This is when the behavior stops completely because it no longer gets any reward at all.
Once the behavior is gone, you want to make sure she doesn't have a need to start doing it again. Going back to the beginning of my thoughts, why does she want to get out so much? Is her cage as comfortable & interesting enough that she can be content to stay inside it?