GCD or insta-cast abilities more important?

[Spoekes]

I've noticed that often when an ability hasnt been used for ages, if I use another one, I can't use the first one either until the second has recharged. Other abilities are immune to this and I can click them whenever they've recharged.

Can someone explain why/how this works? I dont understand why i cant use an ability when its recharged just because I use a completely differemt ability.

can't follow, can you give an example?