The crusting can also be a result of insufficient sand bed fauna/infauna activity. Without a sufficient turnover rate of the substrate a bacterial matrix will form due to bacterial inactivity (which can be compounded when highly saturated kalk solutions and such are being dosed rapidly and calcium phosphate is precipitated).
This is often an occurence in younger systems with immature or "improperly" set up sand beds. Also, aggressive crustaceans like Cleaner and Peppermint shrimp etc. can quickly devastate the fauna population of a young, maturing sand bed.
Check out this thread on RC for a rather lengthy debate on the subject:
Clumping Sand Beds