http://www.google.com/search?hl=en&q=(2+**+32)+-+1&aq=f&aqi=h1&aql=&oq=&gs_rfai=
The database column that holds a user's "thanks" count supports a number between 0 and 4,294,967,295. In computer science terms, it's a 32-bit unsigned integer. If you're at 0 and a "thanks" is removed for some reason, the number will go from 0 to 4,294,967,295 instead of -1.
What probably happened is that a certain condition occurred (e.g., a user deleted their own post, an entire thread was deleted, a user was banned, etc.) that caused his received "thanks" count to be decremented multiple times for a single post of his. Since I see this all over other VBulletin forums, too, I'm assuming it's a coding error in VBulletin or whichever plugin provides the "thanks" capability.