Disclaimer: I do not eat cheeseburgers, even on matzah.
There are a lot of semi-inexplicable phenomena in the Jewish world, particularly on Passover. Notably, rice and beans are completely kosher-for-Passover and staples of Sephardi holiday cuisine but nonetheless, for reasons of dubious and long-expired relevance, may not be eaten on Passover by Ashkenazim. This custom is, in the words of Leviticus, an abomination.
And then there are the nuts who, for reasons of perceived piety and poor education, won't let their matzah get wet lest it miraculously re-bake itself into chametz. This concept is known only by its Yiddish name, gebrochts, because it is so silly and useless a custom that no other language was willing to waste a word on it. Besides, the notion that what is already risen might yet rise again seems more pertinent to, you know, Easter.
At this point I will NOT make a comment about this explaining why Chabad types are so careful not to eat gebrochts. Err... darn it, that just kinda leaked out.
Anyway, the preceding ridiculousnesses are primarily orthodox Ashkenazi ridiculousnesses and are less interesting than they are peculiar and dumb. What is truly fascinating -- what is to my mind the most inexplicable of semi-inexplicable Passover phenomena -- is the way in which less-religious Jews relate to the holiday and its food rules.
A case in point, which kinda sums up the entire spectrum of secular approaches to the holiday: My secular Israeli boss is particularly partial to cheeseburgers but, on Passover, won't eat the bun.
On a halachic level this actually makes sense, since the rules about chametz on Passover are arguably stricter than the rules about not eating [cow] meat [topped, not cooked,] with cheese [made of the milk of a different cow that was, in any event, most likely not its mother].
And yet it's absolutely remarkable, not least for its regularity, that so many Jews who otherwise do not keep kosher at all suddenly, and semi-inexplicably, and often very strictly, begin to do so when Passover rolls around.
Speaking of Passover rolls, they're gebrochts, and they're awesome.
Lest anyone get the wrong impression, I am not belittling those who follow some rules and not others (like, for example -- since honesty is the best policy -- me and you and every other good Jew I believe I've ever met).
I just think it's infinitely interesting, and completely cool, that so many different kinds of Jews continue to have such strong traditions in common. Even if I don't really understand why.
As for me, some day I will eat rice at my seder(s). But right now, I have two more days to crumble matzah into my chicken soup and I'm gonna make the best of them.