"ubiquitous" starts with the sound of "you-biquitous" and so the suffix -n is a duplicated non-vowel. ("y" is probably a vocalic glide, but still not in {a,e,i,o,u}.)
I bet the real rule is some reality about feasibility of pronunciation, even though native english speakers see the rules explained in terms of spellings.
The rule as often given in English classes is to use "an" if a word starts with a "vowel sound", rather than starting with a vowel. So, it's "an herb" (since the 'h' is silent in (American) English), but "a ubiquitous".
Relatedly, you can infer whether someone likely pronounces "URL" by spelling it out (like "you are ell") or as a word (like "earl") based on whether they write "a URL" or "an URL".
By way of example, the name Herb is commonly pronounced with a non-silent H, so even in a dialect where "herb" is pronounced "erb", you'd write "A Herb picked up an herb.".
YUP! Sorry I don't have a good citation handy, but lots of English grammar happens as a result of misaligned word boundaries. "napron" (from the French naperon) became "an apron". Orange (from the Arabic naranj)... "an ewt" became "a newt". etc.
A urinal
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