What causes "spurrs" in feet?

Depends: On where spur forms. Heel spurs form from excessive tension on attachment of either plantar fascia or tendons that attach there: usually from inadequate foot support or tight structures. Other spurs form from excessive pressure causing bone to bleed and form spur. Some times an injury can cause a spur to form.

Very irritating.: Typically caused from an irritation to the surface of the bone. This irritation caused bony growth. The variable is the type and degree of the irritation.