The story
The irregular form snuck originated by analogy with struck for the past of strike, stuck for the past of stick, etc. Snuck was originally limited to a few dialects, but is now very widespread (especially in American English) and is recognized by most dictionaries. The word is now one of the best examples of irregularization of a regular verb, along with dove.
Etymology from Wiktionary, CC BY-SA 4.0.