When living in contact, hunter gatherers and farmers are in natural conflict since the most concentrated food source you could gather (or hunt) from is a farm. Raiding must have been fairly common.

In a conflict like this, it seems natural that the farming community would prevail, since they would naturally be more motivated to do so (this is their primary food source, they are the ones being attacked), and as a stationary community might be expected to build up more resources and be better prepared to defend themselves.

Perhaps there wasn't a change in culture, but rather that farmer tribes were able to grow bigger? A hunter-gatherer tribe could be size-limited according to the available resources. Likely 2 neighbouring tribes would therefore be roughly the same size. Which would make it hard for a tribe to fully exterminate other tribes. Would you send all your fighters to take on another tribe, with the risk of none of them coming back, leaving you defenseless? What if a dangerous animal walks into camp while all fighters are away?

If a farming tribe could sustain double population, that would mean double the amount of fighters, thus it would reduce the risk greatly of sending out a large group of fighters.