Strange Encounters

We went out for my mum's birthday last nite. We just went to a local restaurant in Hurstville, then decided to get some dessert. Being the cheap Asians that we are, we ended up at our local Coles, picking up a few groceries along the way. Anyway, my dad was in charge of the trolley, while the rest of us started puttering around the fresh food section. We naturally convened, I think we were in the vegie department, when I looked around. Mum, sister, dad, no trolley.
"Dad, where's the trolley?"
He looks at me blankly, blinks once or twice, then turns around, looking for the trolley. There are a couple of them strewn around the place, each with a few items sitting in them. My dad takes a step towards them, to seek the empty trolley that he had abandoned only a minute before.

There was a man - probably in his 50's, with a crew cut, polo shirt, shorts and sandals - who was standing around. He must have overheard us, because when he saw us glance in the direction of his trolley, he stepped up, pointed at the trolley, pointed at himself and then uttered a single menacing word. "Mine".
My brain was suddenly overloaded with potential comebacks and pithy lines (well not really - it was overcome with the fact that a good response would have been really funny) as the man proceeded to push his trolley away to the next aisle. I think my dad was equally as stunned by this little incident, but as we looked around we realised why he had responded so strongly.
"I think that guy stole our trolley!"

To this day (ok, it's less than 12 hours later) I still can't work it out. Who enters a grocery store, ignores the plethora of trolleys sitting just outside the entrance and then steals an empty trolley - and when the potential of being found out arises, proceeds to engage in intimidation tactics?