They tend to tier things so business travelers will pay more, so there may be fare rules that increase the cost dramatically the closer you get to departure date, or charge more if you do not stay a certain number of days (I've heard one week as well as requiring a 'Saturday stayover')

Of course business travelers are the ones most likely to research and plan out how to get around these, so they might book overlapping tickets pointing opposite directions and interleave them. Airlines get upset about this, but they also know that if they try to crack down that these travelers will just take their (substantial) business elsewhere.

Especially if you are doing like a two day trip that looks like it might be business. E.g. LHR to JFK Mon to Wednesday.