Interesting change in design. Some bosses still have some randomness elements to them (like Hivemind) and others are almost exactly choreographic (Queen Azshara Mythic). I believe each fight tests different kinds of skills, making a raid more engaging.
This happens because someone killed the cat. If you do the whole raid keeping Mr. Bigglesworth alive, then Kel'Thuzad will barely cast any spell. Kill the cat and face his wrath.
It's the same way with Twin Emps teleport as well. Doesn't always get cast on cooldown so sometimes you will (as melee) run across the room and have to wait/kill bugs until they switch.
I love these "Not a Bug" reports from Blizzard. They give so much insight into the inner workings behind the game engine and the devs who respond always come back with honest, forthright explanations as to why things that seem like bugs are actually working as intended.
Why would anyone kill the cat? That doesn't make sense..
The spells that can delay Frost Blast are Shadow Fissure and Frostbolt (the aoe one). They all belong to the same cooldown category "Creature Special" (ID 1152). Frost Blast and Shadow Fissure trigger a 5 sec category-wide cooldown, Frostbolt - an 8 sec.Chains of Kel'Thuzad also belongs to the same category, but it's a self-cast script effect and I can't be certain who casts it - whether it's KT or the players that are chosen for MC. It probably is KT, tho, as it's the only spell that I can see that could be used as a "primer" for target selection and casting of the actual MC Chains of Kel'Thuzad. It doesn't trigger a cooldown, but it should still be subject to cooldown from other spells in that category.