That leading politicians often leave office (more or less voluntarily) after an electoral disaster is nothing new, but pre-emptive resignations like those we see in the UK strikes me as being rather unusual.
The motives behind the resignations would be interesting to look into. Are ministers leaving now because they want to minimise the public humiliation of being hunted out of office (even if it is by proxy, if we assume that the EP election is used as a second-order election), or are they hoping that their resignations will make the electoral defeat smaller? I.e. are we dealing with purely egoistic motives or party loyalty?
Needless to say, the ministers may not have been jumping. It is perfectly possible that somebody pushed them, and this would be easier to explain: In the face of a historic defeat, the Labour leadership hauls everybody who are seen as liabilities overboard. The timing is an indication of the size of the disaster.