Where does "wicked" get its /ɪd/ from?

OK, here's my two-cents worth.

(1) There are actually three ways of pronouncing the -ed endings: <ɪd> (near-close front vowel, commonly occurring in urban U.S. dialects), <əd> (unstressed mid-central vowel, commonly occurring in rural North Midland dialects), and (mid-vowel sound, now rare in America but still prominent in many British dialects).

Wicked is pronounced with two syllables because it refers to an animate thing (usually human). Thus it is like legged (a two-legged beast), hanged (a hanged person), naked (a naked person), etc. There are other words which do not belong in this category, as modern English is a very messy language. For example, wretched. But note that wretched originally applied only to humans and was applied to nonhumans over time.

Back to wicked: wicked (wɪkt) is applied to inanimate things/nonhumans, such as a lamp, just as legged (legd) is applied to inanimate things/nonhumans, such as a table. Whether the word ends in a t or a d is determined by the position of the tongue just before the final consonant, just like whether we pronounce an s or a z sound in plural forms.

The interesting question for linguists is how wicked attained its present form, given that it is derived from a substantive form, not a verbal form—probably by analogy with other adjectival verbs, just like modern American English snuck was formed by analogy with cling, clung, clung. Originally it was sneak, sneaked, sneaked.

There are other words in this category too. /lɜrnd/, as in 'I learned something' v. /'lɜr nɪd/, as in 'a learnèd man'. /bɜnt/, as in 'a bent leg of a chair', v. /'bɜn dɪd/, as in 'a man on bended knee'.