We are not explicitly told why Samson went to Etam, so any suggestion is speculative at best. It may be that Samson went here instead of to his father’s house, lest the Philistines go there and try to burn it down, as they had just done to the house of his father-in-law. So perhaps he is trying to avoid retribution. Alternately, or perhaps additionally, he is still trying to avoid his Nazirite calling. When he previously went down from Timnah to Ashkelon to kill thirty men and get their clothing, he got away with it, for the Ashkelonites never came after him. After his wedding, if he had not gone back to Timnah, that may have been the end of his confrontations with the Philistines, and he would have been free to live as he wished. Perhaps his thinking now is that if he stays hidden long enough, the Philistines will get on with their lives, and he will become a distant memory at a certain point. These are some possible reasons why he goes to Etam.1
8 And he struck them hip and thigh with a great blow, and he went down and stayed in the cleft of the rock of Etam.