Welcome to the board, though sorry for the reason.
Hmmm. Often we will question vision and/or hearing in such a situation. One or both is failing so that Mickey was startled when the woman appeared all of a sudden? You say people petted him in the truck before but when was the last time it happened?
Some other health concern going on? When an action is so out of character you have to wonder if there is a hurt somewhere or maybe not something that hurts but, say, hypothroidism? Which can provoke behavioural changes.
Startled but because of sun shining in his eyes? Startled becasue she appeared from behind you? I know you didn't say that but maybe to Micky it appeared that way?
Worried about a hand not belonging to his owner appearing all of a sudden beside his owner and it appeared to be a threat to his owner, you? Becoming protective of you and his truck?
Is the lady hurt? Is she filing a complaint? Was she frightened? Hopefully this was just some freaky, one off thing that will never happen again but I know you will watch for it. I'm sure some others will chime in soon with ideas.