He may have damaged his tail and then pulled out the feathers cause they were hurting him, if that is the case it isn't much to worry about. The new feathers as they grow in, on my tiel were very easy to break off.. cause he would grow like 2 top ones and without the bottom ones to support them, they would break at the least little thing.

If he has the puffy downy feathers coming out, he may be molting. Do you know how old he is? If he is molting expect a slightly grumpier bird, make sure he has cuttlebone for calcium available at all times and he should grow his tail back pretty quickly.