‘You wait then three come in a row’ – Starmer compares buses to Tory defections

Danny Beals, Labour's candidate for Uxbridge and S Ruislip, introduces himself and humorously compares a bus to Tory defectors, noting that it has a fridge in the back. He jokes about checking for Boris Johnson, who used to be in the area.

What is the context of the statement about Tory defectors?

What humorous suggestion does Danny Beals make regarding Boris Johnson?