I wasnt aware a challenge caused you to lose more exp/skills, if I prove to be wrong there fair enough, but Ive never noticed anything of the sort.
Possibly it's been done because the challenger doesnt want to gain bloodlust from a challengee who is smaller than them, although that wont be the case against you.
Possibly it's done because you very rarely lose dp, and when you do everyone wants a shot at you because some of the staves in thakria are yours and they're irritating (or maybe it's you that's irritating *gasp*)
Possibly it's done because you managed to sneak in a kill against someone and they thought 'Right Im gonna get the bastard for that, how dare he?'
Who knows, none of the answers I can think of are particularly interesting, and most of them consist of; you hit me, so Im gonna hit you back.
Written by my hand on the 10th of Eleuthral, in the year 1045.