I have a guess for why this happens for past participles, with very little evidence to back it up. I do it for past participles such as known, grown, drawn, hewn, sewn, blown, flown, thrown, shown, mown (so mown is pronounced as in Moe 'n Larry). I believe I got this from my mother. Her father grew up in a German-speaking town in the Midwest. In German, all regular verbs have past participles that end with 't', and all irregular ones either end with 't' or 'en' (except for tun, meaning do, with past participle getan). So for a German learning English, it would be fairly natural to pronounce these past participles as if they ended with en.
The pronunciation of pattern may be a completely different phenomenon, considering @djm's comment on the Irish pronunciation of pattern.