Mass appraisal is the mechanism. The city values thousands of properties from recorded attributes and a base date, not from a visit to each one, so the whole neighbourhood can move together even when nobody touched a hammer. When the market climbs between revaluation cycles, every comparable house in your area typically climbs with it on the next notice.
A second cause is a data correction rather than a market shift; if a permit, an addition or a basement development from years ago finally made it into the file, the assessment catches up in one step. That is different from your home appreciating and worth confirming against the property record the City keeps on file.
If the number still looks wrong relative to similar homes on your street, the remedy is an appeal to the Board of Revision within the deadline printed on the notice, not a call to an agent. For what your house would actually sell for today, Joel Dyck's hand-reviewed valuation compares it against recent sales rather than against a tax model.