I'm assuming it's due to the usual problem, wherein the other specs (specifically Affliction) sim slightly higher, so all the really high-end raiders that pull big numbers switch to it. Hence, the apparent actual change in Destruction DPS is massively exaggerated on log-aggregating sites.

The ridiculous pet stance changes affect all specs equally, I doubt that's in itself causing a specific Destruction problem.

Basically heres how I saw it.

1) Glyph of imp was bugged.
It used to give your imp 40% dmg boost and other pets 20% with same glyph.. (so basically it gave all pets 20% dmg boost and 20% top of that to imp specifically).
That time destro was at same level as rest.
2) And the gear was bit low so low haste etc and so forth? Not sure how much having extra ticks on dots matter on grand scale.
3) Affli was buffed later on..shadow mastery (passive) gave like 25% boost rather than current 30%.

So after glyph of imp nerf and affli buffs it basically went down a lot.
Stuff like simulations said to taken account this bug so it was "competitive" just because of that glyph. Also stuff like keeping imp soulfire up "all the time" etc is annoying.

I love bane of havoc a lot but currently for me its no competition even on fights where I could abuse BoH like magmaw heroic or omnitron..or beth/alyz.
Only fight where I can see it still doing well is Four winds first boss - havoc and CoE on one and dps other.

Hopefully I cleared more stuff than made confusing :'D

@mazca13
Good point about the flavor of the month effect. Maybe that is why there are so many one finger mages.
I was under the impression that imps do proportionately more damage for destrolocks than felhunters do for afflocks. So a problem with pet would hit destro harder. I have no idea what the percentage is for demo, but they are stacking mastery which may even things out at higher gear.

@ukwest

From what I recall, since the buffs to Shadow Bite the felhunter's DPS as affliction makes up a pretty similar proportion of total damage to the imp's for destro. I may be wrong though, I haven't got recent numbers to point at.

It's also worth remembering that an imp loses less DPS if it's randomly target switching. If I'm, say, nuking Rageface as affliction and my felhunter decides for reasons best known to himself that he's gonna go attack Riplimb instead, he might waste upwards of ten seconds scampering across doing no DPS at all. The imp, on the other hand, is likely to be able to just swivel and start casting again. His target switch is therefore annoying, but not actually too bad for overall metered DPS. (Obviously far *more* annoying in a pulling-extra-trash scenario, though).

@Ukwest
Too answer the Demo part, pets are the main part of Demo's DPS. Right now the Felhunter is the best pet for Demo, since its shadow bite benefits from the dots you put on your target, if it switches targets at random that can be a huge dps loss. Right now my Felpup can crit for upwards of 80k pulling a good 5-6k of my dps and is usually the highest dmg spell in an overall fight. So for demo locks a pet switching can be a huge chunk of DPS lost.