There is nothing enforcing the mapping of one container to one service so people will have multiple if they find it convenient, or sometimes if they simply don't know the "rule".

A lot of people use containers as light weight VMs. Some use them as not so light VMs, in fact. In that case multiple services in one is practically expected.

We’ve seen container with all sorts of weirdness, where half the service could crash and Docker would never notice, because the supervisor process was still running and that was the entry point for the image.