This pronunciation is disapproved of by some who consider it a mispronunciation, although at least one leading American dictionary considers it acceptable. One explanation for this change in pronunciation is that a familiar phonetic pattern is influencing a less familiar one. Many common adjectives in English do end in -cular, such as circular, molecular, and particular. This kind of analogy often gives rise to metathesis.
The Merriam-Webster dictionary has the following usage comment: [1]
A possible explanation for the pronunciation nucular is the prevalance of other -cule nouns and -cular adjectives in English and science (molecular, etc.) and the rarity of -cleus or -clear words. The Latin word nuculeus or nucleus means "kernel" and is derived from nucula, "little nut" (from nux + ula). "Nucular" would apparently be the adjective form of the "little nut" root; "nuculear" (so far unused) would be the adjective of the alternative spelling of "kernel".