I have a certain sympathy for politicians on Twitter. If they never get human, it’s pointless them being on there imho, as it becomes just another broadcast channel – but if they do engage without running everything past a team of PR experts, their replies can sometimes get massively blown-up by the media and can even lead to them losing their position (although sadly, not in the ever-ongoing case of Nadine Dorries!).
I haven’t checked to see which tweets got deleted, but maybe he just thought better of the conversation in the light of day, and didn’t want it to be seen by people who dropped by his profile in the next few days. Deleting tweets is an imperfect solution, but maybe better than leaving snippy messages up that you wish you hadn’t posted.