One thing that could cause this is that, if you are far enough away from the mountain home, an immigration wave can take so long to reach you that it may be on it's way when you hit the cap.
Also, IIRC, the immigrants are triggered by caravans, so if a dwarven caravan left while you were still under, that could trigger a wave that may arrive after you are over.
And both these can happen, so, in theory, it should be possible to get 2-3 waves after you've hit the pop cap because of the delay involved in the immigrants 'knowing' you are over.
And sometimes, Armok just wants to make you suffer.
And by suffer, I mean build a death machine to offer up blood to Him.