Basically, most of the liberal parties have found this "hack" to win elections. Where they wouldn't be able to win elections normally, they simply keep importing more and more people, and these immigrants reliably vote for these parties. Eventually, these places end up becoming one party states. Consider the case of California as an example.
It's quite an indictment of the democratic processes.
The only thing I could think that would make it seem abnormal to you is the demographic of the immigrants.
It's their ideology and it is up to the democratic process to judge it (favorably or unfavorably). And - although personally I favor controlled immigration only - it is perfectly healthy for a democracy to have parties that defend and present other perspectives.
Like I just said, it's working extremely reliably. Look at California, the opposition is decimated there not by the merit of liberal parties work or argument but by the sheer number of immigrant votes. Granted there is a delay factor such that new immigrants can't vote but the mechanism of immigrants voting for liberal party is still at work.
This is just for state level, I predict something similar happening at Federal level elections in a 1-2 election cycle period. Trump election was an exception IMO.