Multiple Worker Sessions Appearing

I have noticed strange system behaviour when multiple sessions of type 'Worker' are created simultaneously in a customer's Live system. I know that these sessions can be created when a user launches, say a form, where different elements of the form each cause a worker session to be created for that user.

However what is strange about these worker sessions is that they are created for different users (eg 6 at a time), with EXACTLY the same timestamp, and the customer is sure that only one of the 6 users is actually the one logging on. The others are not even accessing the system.

What could cause the simultaneous creation of these 'false' worker sessions?