Spiel, do you use action ID's for your buttons? Do you use custom created ones or do you borrow the default UI ones?
Macro execution is handled 100% by Blizzard. Only thing I can think of is if while spamming the button, for a brief second nUI swaps action ID's on the button and causes another spell to start casting that is not in the macro.
This would require a detailed look at an event log to see if while spamming a spell/ability that would interrupt steady shot does indeed start, then quickly interrupted itself by the spamming of the macro. Otherwise, I have no other theory atm
