to increase in number or amount quickly
verb
http://www.merriam-webster.com/dictionary/proliferation
“Denzil found this out when he attempted, on this most sacred of days, to do the things we could not do because we’d always done them another way, our way — a way we all hated, to be sure, but could not change. Denzil wants to open a present on Christmas Eve — don’t do that, Denzil. Denzil wants to go for a walk — I’m so sorry, Denzil, that’s impossible. We’d like to, but we just can’t swing it. Why not? Because, Denzil. Just because. Because like the two parts of Ireland, because like the Holy Trinity, because like nuclear proliferation, like men not wearing skirts, because like brandy butter.”
She made it clear that the reason they couldn’t go against their traditions or do things differently was because they were so used to it and it’s routine. It doesn’t really need an explanation
