A move like that, likely you and your comparables were in a very illiquid market. Once some comparables hit the market, the assessments move. Could also be zoning like multiplex rules? A lot which had disadvantageous influences, like being on a corner might all of a sudden be a prime target for redevelopment.
This is a concern with assessment, that because of the number of transactions, more 'mass market' properties more accurately reflect the market value, since there is more of a market to track.