The trolleys are in fact slaves owned by the supermarket and they try to escape at any opportunity. So they are chained together to prevent this. Supermarket customers are complicit in this inhumane treatment as they chain them back together again after using them to lug around their heavy shopping.
Edited to add: Aliens plan to take over the oceans by inhabiting the bodies of herrings.
Why are supermarket trolleys sometimes found in canals?
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