There are a few possible causes.
1. Uneven water. As the buds develop, the plant puts out a full effort to develope those buds. If they don't get proper water, they can abort them.
2. Too warm temperatures could cause them to abort.
3. Too low humidity is possible but that would be my last thought.
4. Insects, either thrips, aphids, false spider mites would be the top three culprits.
5. Too strong light leading to the buds overheating and aborting.
