What happened:
The return fuel lines were recently replaced on my 1984 300SD after the mechanic noted a diesel leak.

Post repair, the car was driven for about 10 miles (~25-45mph), before smoke was seen coming from under the hood and excessive temps were seen. The classic (as I now know from the internet) broken radiator neck was found. We replaced the radiator (DIY) but while flushing the hoses (so idling the car) noted leaking diesel fuel. We found the last return line popped off. We assumed we knocked it when messing with the heating block hoses and popped it back on. Took it for another test drive and 30 minutes another diesel leak again. This time the first line popped off, though we noted that several of the other lines had "backed up." As in the lines were going to eventually pop off they just hadn't gotten there yet.

Could this be because fuel filters need to be changed? Are there other potential causes? Is there something that happened from the overheating that is causing the lines to pop off?


Additional Information (possibly irrelevant)
The car went in to the mechanic originally for "groaning" when turning the steering wheel. They replaced the leaking steering gear box as well as the return fuel lines and a rear axle seal. They also noticed a leaking differential, but said we could wait on that repair since we had racked up a nice bill so far.