[dpula2]

Two weeks ago I have noticed there were few swarm cells in my strong hive.

In order to prevent swarms I have removed the queen in the original hive (left there brood with a queen cell) and on top of that I have made two nuc splits with queen cells.

Yet, after all this, today I saw not one but two swarms that came out of that hive within few hours. Why is that?

 

[Akademee]

This sounds like something called an after swarm, though perhaps not in the traditional sense. Your bees wanted to swarm because they felt congested, most likely, and when the new queen hatched, they decided to leave with the virgin queen. This sometimes happens after a regular swarm where the old queen leaves and once the new queen emerges, they swarm again. These after swarms tend to be weaker than the first (prime) swarm with the old queen. When you split colonies for swarm management, make sure you transfer a lot of bees as well.

 

[GregV]

... Why is that?

Because you missed queen cells.