What is the most common cause of tetanus or lockjaw?

One agent only: The spores of a germ called clostridium tetanae are around us in the soil & dust.If an opening in the skin is contaminated by these spores, they can infect the area &produce a toxin that produces tetanus. Immunized people have the ability to avoid tetanus by producing antibodies to the toxin.

You mean trismus: Trismus is a common condition caused by hyper activity of the jaw muscles. This causes them to be dire and stiff thus limiting opening.