Parliamentary systems (like the Dutch have) of government require at least 51% of the population to vote for them.
If one specific party gets 51%, they can do as they please.
On the other hand, if no one gets 51%, then two or more parties can work together to form a government assuming at least 51% of the population collectively voted for those parties.
In this case, the government of the Dutch was a coalition (that together received more than 51% of the vote of the last election) that included the Christian party. Without the Christian party there would be less than 51% of the vote, and thus the government would be illegitimate.
Even though the Christian party was a minority of the ruling party, without their support there would not be the 51% required to form a government.
So when the ruling party proposed changes to a hot-button issue (immigration) that the Christian party disagreed with, when they could not reach a compromise the Christian party essentially withdrew from the coalition that formed a government out of 51%+ of the voters, and thus the ruling party no longer represented a majority of the country, and thus it was no longer a government (aka it collapsed).
The same party can still be a majority of the next government, but they might need to make different alliances to reach the 51% mark if they want to enact any substantive legislation on an issue like immigration.
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