Probably his views on Brexit that cost him his seat. Your electorate have voted unanimously to leave the EU and as their political representative you come out and say we should stay.! Clearly not the right man for the job in Sheffield Hallam as far as they are were concerned.
With a high %age of students and recently ex-students, Sheffield Hallam was probably the most pro-Remain constituency in Sheffield. His majority was slim in 2015 though, so he was always going to be in trouble.
The students don't trust him after the student loans fiasco he oversaw. Glad he's gone the cretinous *****.