This is from rbarr, the resident linguist on another board. It is in answer to a question about shined/shone but the principle is the same. Reprinted with permission.

The course is "Structure of English," and we've been covering issues like this all term. In my last lecture, in fact, I discussed the creation of new irregular past tenses, like pled, snuck, dug, stuck, knit, all of which used to be regular verbs: pleaded, sneaked, digged, sticked, knitted. It's fascinating, Commentator, that you think "they just don't sound right." It shows that there is more going on in the mental lexicon than just rules for regulars, vs. memorized lists for irregulars.

The problem with shined is a little different. Historically, causatives of intransitive verbs were automatically regular, even if the base verbs were irregular. This is the source of the distinction between, for example, intransitive lie, lay, have lain (irregular), and regular transitive lay, laid, have laid "cause to lie."

This is also the source of the difference between hung and hanged. Originally, the intransitive was hang, hung, have hung (irregular) : "The wet clothes hung on the line." The regular transitive was hang, hanged, have hanged 'cause to hang,' as in "They hanged Danny Deever in the morning." However, the identity of the present tense between the two verbs hang has caused the regular transitive hanged to die out and be replaced by the irregular hung everywhere except in the special case of executing a person. So now one can also say, transitively, "They hung the wet clothes on the line."

This is the current problem with shined. The intransitive has always been irregular: shine, shone, have shone [pronounced 'shon' if you're British or Canadian], as in "The light shone from behind the stained-glass window." The transitive would have been regular: shine, shined, shined 'cause to shine', as in "He shined his shoes." There has been some mixing of forms, though. You still can't say "*He shone his shoes," but many standard dialects now permit the irregular form for light: "He shone a light through the window." Now we see that shined is crossing over into the intransitive use as well: "Never have two women shined so brightly."