Within a few generations, names might be changed to avoid potential stigma associated with belonging to a certain ethnic group. This occurred with many German families after world wars one and two. There is also the possibility the names could simply not be translated due to discrepancies in grammar and punctuation between the immigrant's native language and English.

Anecdotally, I know it is quite common for exchange students (particularly from East Asia) to adopt English names entirely separate from their birth names. It should be noted that the idea immigrants were forced to change their names upon landing is a popular myth.

If it's anything like Irish names being anglicised, it's because maybe the immigrant was illiterate and the clerk not speaking the immigrant's language. There'd be quite a bit lost in translation. Hence O'Reilly could become Riley or Reilly.

Similarly for traditional Polish names like Kowalski/Kowalska, you have a male/female version. Any Pole will know Ania Kowalska and Pawel Kowalski are related but the naming tradition doesn't translate well with bureaucracy!

These stories will be different for almost every family. As another poster pointed out, the "changed at Ellis Island" trope is a myth, and names were usually changed by immigrants themselves later, for a variety of reasons. If you can talk to some of the older members of your family, you might find out why your ancestors decided to shorten their name in the way they did. In my family, I was told that my great-grandfather changed the spelling of our last name to make it easier to pronounce for the native English speakers he was working with when he was working on the railroad in Saskatchewan. I have no way of verifying that, but often family stories like these are what we have to go off of for the "why" behind immigrant name changes.