The Culture Channel

I'd put the anti-canon-het impulse down to two things, both ship-war hatred of seeing a contradictory ship become canon and the fact that canon het relationships can often feel tacked on or otherwise badly-done, either because the canon writers don't bother to actually "show their work" for the relationship ("he's the lead actor, she's the lead actress; it's inevitable that they'll fall in love, why would we need to put any work into it?") and have it come out of nowhere, or because the female character involved is either a two-dimensional Designated Love Interest or an inconsistantly-written borderline Sue (the fact that female characters are much less likely to be forgiven for being badly written or two-dimensional than male ones are is a whole different issue).

Then, of course, there's the classic "hetting canon up" reaction TPTB have been known to have if/when they realize their show features a popular slash pairing, which slashers loathe and rightly so - though I don't think it's quite as common as some slash fans claim. I'd put random het introduced out of nowhere in later seasons of a show down to the plain old heteronormative "It is a truth universally acknowledged that an unattached male protagonist must be in want of a girlfriend" outlook as often or more often than I would to deliberate attempts to send a writerly "fuck you" to slashers.