#1: Does it have an adjustable trigger - as in, have you pulled the action from the stock & looked for adj screws ?
If yes, are the screws sealed ? If no screws, then standard gunsmithing methods would apply.
If it has screws, an inspection should tell which is weight-of-pull, or sear engagement, or overtravel. (There's usually only either two or three screws in adjustable triggers)
If you're not very familiar with adjusting triggers, doing so hap-hazzardly can lead to an unintended discharge and/or death, so please don't try to "wing it".
If not adjusted properly, and in the proper sequence, any trigger may produce such an event - even though it "seemed to work OK" for the first several times.