The definition of conspiracy theory is: "a theory that explains an event or set of circumstances as the result of a secret plot by usually powerful conspirators" (https://www.merriam-webster.com/dictionary/conspiracy%20theo...)
So, here we have an event - payment processors refusing to serve Gab. The theory is that this is explained by from some entity "above" that we don't know anything about and that clearly acted in secret to force them to do this.
Using the dictionary definition of a conspiracy theory, this is a conspiracy theory.
Oh, hush.
"Conspiracy theory material" has a strong connotation that a theory that explains an event or set of circumstances as the result of a secret plot by usually powerful conspirators doesn't capture at all.
The connotation heavily implies that the theory is to be disregarded as ridiculous and false.
But you know that already, so why am I even telling you?
Sure, it could be distributed social pressure, but even that would just be a standalone event resulting from a larger and more subtle push from above to use social pressure as a mechanism for modern information gatekeeping.
Like I said, I'm not speculating as to who or what is behind this push for modern censorship but don't delude yourself into thinking it isn't there.