[FrankenMauser]

A: Because the trigger has been improperly adjusted.
B: Because some one botched a "trigger job".
C: Because the stock is improperly inletted, causing a trigger malfunction.
D: Foreign debris in the sear/trigger. Clean it, thoroughly.
E: The Sear and/or Trigger is severely worn, and no longer properly engaging the other.
F-Z: other posters will provide other possibilities.

Above all else: Take the action out of the stock, and clean it thoroughly. See if the malfunction occurs while out of the stock (before and after cleaning). If it does not, see if the malfunction comes back while in the stock (after cleaning).

It's not a head space issue.