I believe the State mandates by law that homes must be reassessed every 3 years. If you've been paying the same tax amount for, let's say 8-10 years, then you weren't paying enough based on what your property was worth.
There would have to have been LOTS of these insane sales in your neighborhood to force such an increase in your assement. I don't believe there were because our market is pretty stable and can't be compared to what the parts of the rest of the country is doing in terms of predatory lending and the housing recession. So basically, there was either a large improvement on the property or you weren't paying enough, based on the market value, for an extended period of time.

