What else could it possibly be? You think PayPal pulled your name at random and decided "Okay we're just going to fuck with this guy"?
I'll say this though. If you refuse to link a bank account, that's going to be an ongoing cause of annoyance.
I have a separate account for internet stuff with no overdraft protection. Only once did I make a withdrawal against it without first transferring the funds to cover it. I called my bank and asked them to waive the overdraft fee on account of my long history with them. They were kind enough to do so.