It's a buildup of fatty tissue due to the protective sheath around the nerves being damaged. The fatty tissue is trying to protect the nerves from further damage.
I have it and I'm dealing with it. Have your pro-shop shape the top of the thumb hole to match the "lump" at the base of your thumb to relieve the pressure. It might not be perfect, but the lump should go down in time otherwise take some time off away from bowling, but the thumb hole will still need to be adjusted to take pressure away from that spot so the issue doesn't return.