Phonologically,« lire » and « rire » sound like a minimal pair, with the first letter as the only difference. So what might explain the difference between their « participes passé »?
Their etymons are Latin.
Reply to comment by user 'sumelic': I'm inexperienced in Latin; so please feel free to edit.
lire originates from legō, whose active present infinitive is legere.
rire originates from rīdeō, whose active present infinitive is rīdēre.
past participle
? Please edit my post.