It's just an incorrect tense construction of the verb that passed into common usage.
Verb tensing requiring a change in vowel is among the hardest area of grammar to create hard and fast rules for. The majority of past participles just add "ed" (or sometimes just "d"), such as walked, soaked, etc.
However, when that doesn't work there are really no good rules to say how the vowel should change. There are also some distinctions originally made between various past tenses that have been lost in colloquy; for instance, different conjugations ("it stank", but "I stunk") or between various past tense constructions resulting in passive/active voice differences ("I sang", but "the song was sung").
"Sneak" is a verb that is technically regular in past participle formation ( add -ed to form "sneaked"), but because it is phonetically similar to some exceptions that change the present tense vowel to "u" (stunk, sung, sunk), the vowel change is becoming acceptable for this word as well. Gotta love the organic nature of human language.
EDIT: Further exercise of my "Google-fu" has brought up this blog, which in turn references this BBC article. It appears that the word "sneak" is the latest of a series of verbs that have undergone "weak to strong drift".
To explain (hopefully consicely): early Anglo-Saxon language categorized verbs in several classes depending on how their various forms were constructed. This ancient system is where we get such peculiarities of tensing as bring/brought, think/thought, see/saw, fly/flew, and sing/sang/sung. Over time, the various verb classes coalesced into two: verbs that formed the past participle by simply adding "-d"/"-ed" were "weak", and verbs that formed the past participle any other way were "strong".
It is unusual but not unheard of for a verb's conjugation and tensing rules to change. However, when it does, it's usually "strong to weak"; conjugation and participle construction are simplified from the complex vowel-changing rules to easy suffixing. The word "glide" used to have the past participle "glad", which has been completely abolished in modern usage in favor of the "weak" construction "glided".
However, "sneak", and words that have gone before it like "dig", "string" and "dive", went the other way; they went from simply adding the suffix to changing the vowel. "Digged" became "dug", "stringed" became "strung", and "dived" became "dove". In this same way, "sneaked" is becoming "snuck"; the word "snuck" is already generally accepted as "sneak"'s past participle in most of the English-speaking world except for Britain. Eventually, it is thought, those tea-timers will give in to the pressures of the colonies.