Hi Siebe,
I’m not sure whether the Sakyans massacre had been explicitly explained in any sutta. But I do believe everything happens for a reason (or reasons). Some people are born into good countries and families while some who are not so fortunate do not even have a chance to encounter Buddha Dhamma. Everything in this world is conditioned things or conditioners or both conditioned things AND conditioners. Take for example this kammaja kaya of ours. We have this body because of past kamma. Thus, it is a conditioned thing, a sankata. Everyday, we are making use of this body to do volitional acts, sankhara, to condition other things. This kammaja kaya is a breeding ground for many impending kamma beeja to ripen and let nature impart kamma vipaka onto us. A seemingly simple decision of buying an air ticket and boarding an aircraft to go on a holiday is already acting as a pre-requisite for past wholesome or unwholesome deed to ripen. Coming to Kama Loka and to be born as a human being alone is already a condition for nature to impart merciless unwholesome vipaka onto some beings. This is most evident in birth defects.
This burdensome body is like a time bomb waiting for the right trigger for something to happen. We cannot fully discern what sankhara to drop so as to avoid unwholesome vipaka. Being at the wrong place at the wrong time are conditioners for mishaps to happen. But they do not happen by randomness. Things do not just happen. Things are made to happen. All that we are experiencing now are just vipaka of kamma performed deep in our past or in this very lifetime.