Quite possibly because they were Jewish. Sometimes it was only the dropping of "berg" or "stein" from the end of their name but it often meant changing it altogether. Jacobi to Banta, Shereshevsky to Sherry and Cohen to Craig, these are all examples of surname changes. We are given a variety of reasons for why it was done, it's often blamed on an immigration clerk and sometimes this is true but Jewish surname changes were quite frequently made because they wanted the same opportunities for their families as other Americans. It's quite shocking for descendants to become aware of the original Jewish surname, they often had no idea.
Suellen Ocean is the author of Secret Genealogy II a how-to for tracing ancient Jewish ancestry available at Amazon, B&N and Ocean-Hose.