There's actually a pretty good reason to mispronounce "nuclear" as "nucular". The Latin word "nucleus", meaning "kernel", is basically a mispronunciation of "nuculus", meaning "little nut". And "nuculus" is a much more typical Latin word construction than "nucleus".
So from one perspective, the people saying "nucular" are using their well-tuned intuitions about Latinate words to correct a very old mispronunciation, rather than accidentally creating a new mispronunciation.