According to A Critical Pronouncing Dictionary and Expositor of the English Language, 1839,
The adjectives naked, wicked, picked (pointed), booked, crooked, forked, tusked, tressed, and wretched, are not derived from verbs, and are therefore pronounced in two syllables. The same may be observed of scabbed, crabbed, chubbed, stubbed, shagged, snagged, ragged, scrubbed, dogged, rugged, scragged, hawked, jagged; to which we may add, the solemn pronunciation of stiff-necked; and these when formed into nouns with the addition of ness, preserved the ed in a distinct syllable, as wickedness, scabbedness, raggedness, &c.
This explanation is reaffirmed in the book, Teaching Pronunciation: A Reference for Teachers of English to Speakers of Other Languages, 1996,
Whether they function as verbs or adjectives, most English words ending in the -ed suffix follow the same phonological rules as the paste tense inflectional ending (e.g., striped
/d/). There are, however, historically based differences in pronunciation between certain formed ending in -ed, depending on whether they function as adjectives or verbs. The -ed adjectives in this category have an extra syllable and take the
/ɪd/ pronunciation, whereas the verbs simply take
/d/, following the rules for the regular past tense and regular past participle outlined earlier:
[Table with examples comparing the pronunciation of verb forms of words such as blessed, beloved, learned, dogged, and legged, with their adjectival equivalents]
Sometimes, even when there is an adjective with no corresponding verb, the adjective is still pronounced
/ɪd/ (naked, wretched, rugged, wicked).