I had a male Am Staff that did the same thing. We moved his crate into the spare room, and taught him to shut the bedroom door behind himself. He never got over it. He was a foster, so not knowing his history made it difficult to determine if he was crated with dogs is the same crate, and that’s what caused it, or just a random fluke. He didn’t guard anything else, toys, food, treats. Just his crate. He got along great with the other dogs, he just didn’t want them near when he was in his crate.
Sorry, I can’t offer any actual advice, just that others have been there before. He was never trained out of his issue, we just managed it by moving his crate and training him to shut the door. I eventually rehomed him to a couple that didn’t have any other pets in their home, and he still wanted to be in their room with the crate, with the ability to shut the door behind himself when he wanted a break.
Their crates are generally where they go to feel safe when at rest. He may feel threatened by the “alpha” in your home, and lashes out so he can rest comfortably knowing the dogs won’t come near him now that he does this. Just a thought.