Hi Purusharth Verma,
1.if their is any external interference.
2.Distance from source to target cell. Might be some times saples are taken from too far distance by which a mobile can try to H.O & it causes H.O
3.Check Link balance reports, might be imbalancing.
4.Finaly you can do one thing, just delete the neighbour & recreate them with fress parameters. It will works many times.