If a parent record has a roll up summary field, let’s say to calculate the number of line items on an opportunity, if a new line item is added (and hence the value of the rollup field is changed), will triggers be fired on the parent opportunity record? And if so, are there any deviations from the standard trigger order of execution?
The roll-up summary field documentation indicates that workflow rules and validation rules will fire on the parent object, but doesn’t mention what the trigger behavior is.
A similar discussion went here and you may find it helpful.
To answer your question The Trigger will fire. The Document mentions that the Parent record goes through the save procedure, meaning that the trigger will fire on the Parent record if there is a modification on the field of the child record that feeds to the parent as a roll up.