Hans Gruber: Who are you then?
John McClane: Just the fly in the ointment, Hans. The monkey in the wrench. The pain in the ass.
John Boehner's criticism of Ted Cruz and thumbs up for Donald Trump should send a signal to 'anti-establishment' Republican types. Cruz is disliked because he has been a fly in the ointment of the mainstream Republican agenda. Ideologically, Trump philosophically operates far closer to the GOP's position than the establishment cares to admit.
To the extent that Boehner can be viewed as a symbol of what's wrong with the Republican Party, then his criticism can be seen as an endorsement of Cruz's approach for those voters looking for real change.