Give a reason why Joan of Arc was accused?

The bishop accused Joan of both witchcraft and heresy. However, Joan defended herself quite well and he could not prove the allegations. In the end he found an obscure mandate in the Old Testament that forbade women from donning male clothing. He used that to convict her of heresy.

The main reason she was accused of anything was Joan had led the French to several military victories that had seriously embarrassed and humiliated the British army and they wanted her dead.