We have the same problem. It is because the plant is a sport from a thorned type and so occasionally reverts to the original. The seed is definitely spread by the birds, usually blackbirds in our garden. You can tell them from ordinary brambles as the leaves are somewhat different.

Same with me...I also moved mine and more came up in the old bed than the moved plant and root...all with some form of thorn. Can be a hit or miss with fruit flavour I find.

probably after all birds love any plant that offers food and protection.not sure how to stop it though mabye introducing something that scares them away?

Just dig up the thorny ones as soon as you see them. the roots get a long way in a short time!

Thanks for all the suggestions. I am now in the process of layering a growing tip so that I can pass on a growing plant to my daughter.

Lucky daughter!