-ji- is actually the reflexive infix, as mentioned briefly in the Tips & Notes. It is used when the object of a transitive verb is the same as the subject. Therefore it means oneself, itself, etc. It doesn't change and is the same for all noun classes, and placement is the same as the other object infixes. "kukwaa" means "to trip (someone)," so "kujikwaa" becomes "to trip oneself" = "to stumble!"
Other examples of this are kujifunza (to learn, or literally to educate oneself), kujisaidia (to go to the bathroom, or literally to "help" oneself), kujitolea (to volunteer, or literally to put oneself out there), and kujiua (to commit suicide, or literally to kill oneself).