Secularization among Jewish immigrants?

I’ve gone down a rabbit hole recently trying to make sense of my own sense of spiritualism while also trying to understand more about my family lineage, the best-defined component of which is the 1/4 Lithuanian Ashkenazi. I grew up in a diverse community and was raised in what I might call an aggressively secular household (by parents raised in similar households). I had dinner with some friends a few weeks ago and it ended with havdalah, and it just sorta felt like a normal, familiar thing with people who shared a lot of the same values and ideas. I found myself singing songs I didn’t know I knew (but must have learned at some point). It was a bit uncanny.

Anyway. At some point– I don’t know why or when- the Jewish line in my family went from a family portrait my mother described as “straight out of Fiddler on the Roof,” to my great-grandfather and my dad’s family later seeming to have led totally secular, non-religious lives. My great-grandfather wrote about how his father dressed “like a Russian,” and about growing up as the Jewish kid on the block, going to shul, etc., but it just seems weird to me how distanced he seemed to be from it in later years. I suspect this may have been because my family suffered some political grief during the McCarthy era– probably both for being Jewish and for being involved in some lefty causes, and I also know that my great-grandfather worked on memorializing the Holocaust in the 1950s (when it was considered a somewhat controversial topic of discussion, and one that was off the radar of most Americans).

It’s the process of secularization that I don’t quite understand. I realize that a lot of American Jews are quite secular, but I also don’t have much of a reference point for the process through which some immigrants secularized (whether as a matter of assimilation, intermarriage, or something else?), while others developed strong communities. Point of comparison: I’m absolutely certain I’m distantly related (fifth cousins, maybe?) to some people in my hometown who are very much Orthodox.

Was it a matter of trying to assimilate and avoid antisemitism or exclusion? Or is this more common among some immigrants than others? I guess I’m trying to see if anyone else has a similar story– I feel like my own family history involves all of these complex threads that I feel the need to unravel and see where they lead, both as a matter of historical research but also as a matter of me pursuing my own truths as far as religion or spirituality. I appreciate any feedback and I genuinely appreciate a lot of the thoughtful commentary I’ve seen to the Confused, Questioning, or New To This Whole Thing folks on this subreddit.