Four of the ten by-elections in this Parliament, so far, have resulted from impropriety (3 Conservative, 1 Labour). In the 2017-2019 Parliament, three of the five by-elections resulted from impropriety, and in the coalition parliament (2010-2015) some six of the twenty-one by-elections could be construed as having resulted from impropriety (half of them Labour MPs).
As you say, most by-elections result either from death or from people leaving for pastures new. I suppose the reason that there may appear to be greater cases of impropriety at the moment is that this Government has been mired in a culture of sleaze, yet very few cases lead to by-elections. In that respect, it’s rather similar to the Major administration from 1992-1997, when 16 of the 18 by-elections were from deaths, with Milligan’s auto-erotic asphyxiation the only one sleaze related. Yet, everyone remembers that as a sleaze-ridden Government.