First of all, may the souls of all our soldiers rest in peace.
Many possibilities come to mind, but it's best to wait for the results of our government's investigation. The possibilities that come to mind are:
1- Maintenance error or negligence,
2- Sabotage of the aircraft before takeoff,
3- Possibility of being shot down,
These options carry many possibilities in themselves. This is a truly tragic incident. Necessary steps must be taken against those responsible. The root of the problem must be addressed.