This is how they reproduce, as reported by trouser quandary:
After mating with other trolleys using the lock and chain arrangement, a shopping trolley will wander off down a supermarket aisle on its own, acquiring the necessary shopping needed for the next stage of the breeding cycle. When the trolley feels it has enough shopping, it will just stop in the centre of the aisle and will remain there seemingly unattended and abandoned, but in reality it is a kind of pupae stage.
A few days later, the trolley and its shopping will seem to have disappeared. In its place will be a small herd of shopping baskets
For a learned and scholarly explanation of these phenomena, read the whole thing.