Usually it's caused by repetitive action such as perhaps using sheers to prune bushes and trees, or using a certain tool to do woodwork. Think of what you have been doing a lot of. Knitting? Crocheting? Needlepoint? Artwork, etc.?
It could very likely be Dupuytren Disease.
Dupuytren's disease is an abnormal thickening of the fascia (the tissue just beneath the skin of the palm). It often starts with firm lumps in the palm. In...