The license key gets tied to certain system internals upon first use, and is checked against the existence and value of the same internals in subsequent checks. Apparently, toggling UEFI (and/or Secure Boot) modifies one such internal.

EDIT: license -> license key

And then you call Microsoft on the phone and they issue a reactivation, or whatever they call it, and everything is fine. Unless this works differently with OEM versions of Windows (different, more obtuse support people?) than it does with retail, I can't imagine it's actually a problem.

Most people don't realize this.

Don't confuse the technology with the legalities. "The license" is a contractual right that you have legally purchased. If Microsoft's piracy-detecting hair-triggers are firing in a situation where you still have a license per the contact - oem licenses are usually "on this PC" or the like, so toggling UEFI alone shouldn't breach the terms - that should be their problem. You're entitled to your license.