Unusual meaning of "head over heels"

Solution 1:

None of Webster, ODO, Collins, or MacMillan lists "At top speed; frantically" as a possible definition of "head over heels". They only note the two definitions that the OP is familiar with. However, I can imagine without any great difficulty that frantic top-speed running could well involve chaotic stumbling and tumbling, and vice versa. So, it's not all that much of a stretch.

He looked scared, and that was no wonder, because head over heels after him came five other boys.

This could easily have been written by Stephen Fry with wholly different connotations.

Solution 2:

The intention seems to be something more along the lines of the boys in pursuit are falling all over each other trying to catch up with that which they pursue. They can't keep their feet for the speed and willy-nilly fervor with which they are running.