I was wondering if it could be possible that certain spawn conditions are connected to instance / bosskills.
I started wondering after reading http://www.wowwiki.com/Weather_effects
Note: A sandstorm starts automatically when you engage Ossirian the Unscarred in the Ruins of Ahn'Qiraj and does not stop once the encounter is over.
My theorie is that Scourged Whelpling spawns if you kill an undead dragon raid boss. Minfernals if you kill Chromagus in BWL, Silithids for AQ20 or AQ40, ...
The new raid pets would encourage people cleaning up the instances and thus make these connected spawns more likely.
This is just a wild guess, but could explain the long respawn times.