I've just learned that Patch 5.1 added a new /stopspelltarget command that's functionally identical to this AddOn. This means the AddOn is now redundant and you can replace /click SpellStopTargetingButton with /stopspelltarget in all of your macros.

The command accepts macro conditionals, so if for example you wanted to only cancel the blue hand cursor in combat, you can use /stopspelltarget [combat] in your macro. My /click version of the command could also accept macro conditionals, but I never publicised this.

I'm actually not too sure what the cause is, since I've never experienced the issue myself. I just wrote two lines of code that utilise Blizzard's SecureActionButtonTemplate to cancel any blue hand cursor, regardless of the cause.