Geesh, even when dogs are playing the sounds can get me rattled, especially if I can't see, don't know they are playing. I'm sure you recognized Mikey's voice, so I can well imagine you were upset.

Glad to hear no major injuries.

I don't have any ideas to offer. Though it did occur to me, "would not have happened if she were there," as someone has said. Was the Westie owner the one you asked to watch Mikey while you were outside? Ask a different person another time, see if that makes a difference.