This seems to be two separate phenomena, with different origins.
From the other answers, in some Irish dialects, words ending in consonant clusters such as -rn and -lm have the n and m pronounced as a separate syllable. This may be a hold-over from earlier English pronunciation. Apparently Shakespeare sometimes spelled "film" as phillum, so it appears he pronounced film this way. See the comments for this Language Log post. (Although it's possible both the one- and two-syllable pronunciations were used in Shakespeare's time, and he chose the two-syllable one for the sake of the meter).
The other phenomenon is restricted to past participles. I do it for past participles such as known, grown, thrown, mown, hewn, strewn (so mown is pronounced as in Moe 'n Larry). This is pure speculation on my part, but it's possible that this originated with German immigrants—in German, irregular verbs all have past participles that either end with 't' or 'en' (except for tun, with past participle getan). So for a German learning English, it would be natural to pronounce these past participles as if they ended with en.