It was not uncommon. It probably wasn't super-common, it's not like it was an everyday occurrence, but there are a number of recorded specific examples, and more can be inferred.

In terms of numbers, a number to give you a rough idea of things is that Lodz was 36% by ancestry but 33% Jewish by religion, so under 10% of ancestral Jews did not practice Judaism. Being a city, Lodz would be on the higher end, and not all those not practicing Judaism would've converted to Christianity.

As for why, there are a few reasons. It's hard to determine exactly, since not every convert explained themselves (nor could we always trust their word for it on their reason). But some potential reasons can be determined.

First, missionary efforts. Christians, aided by the government in some cases (for much in the 19th century there was no Polish government. This is further complicated that in the internal Ashkenazi Jewish groupings, "Polish Jews" and "Lithuanian Jews" and "Galician Jews" are from an area that does not correspond with that government from every era, nor does it correspond exactly with the local non-Jewish ethnic groups), attempted to convert Jews. For centuries by this point Christians had translated Christian texts into Hebrew and Yiddish, and were attempting to disseminate them.

Second, legal and economic incentives. Jews were subject to a number of repressive laws that limited economic freedom and freedom of movement for Jews. These could be a powerful incentive to convert.

Third, general societal acceptance. Many Jews were relatively assimilated, and wanted to be accepted by Polish society at large. To do this, conversion would've been necessary to escape discrimination and to be fully integrated.

Some examples: Jan Gotlib Bloch was a Polish industrialist. He was born Jewish, but converted to Protestantism. While I'm not sure why, he did maintain some level of positive feeling with the Jewish community, both through his charity to the community and through lobbying on behalf of the community in government (though I'm also not sure how he was viewed by Jews).

A more familiar example from fiction is from Sholem Aleichem's series of short stories Tevye the Dairyman (which was later adapted into the American musical "Fiddler on the Roof"). In it, Tevye's daughter Chava converts to Christianity to marry a Christian man. This would indicate that conversion to Christianity for this reason was not unbelievable to a 19th century Jewish audience, though obviously the specific instance is fictional.

Sources:

• Carlebach, Elisheva. Divided Souls Converts from Judaism in Germany, 1500-1750. New Haven: Yale University Press, 2001.
• Dubnow, Simon, and Israel Friedlaender. History of the Jews in Russia and Poland from the Earliest Times until the Present Day. 3 vols. Philadelphia: Jewish Publication Society of America, 1916.
• Haumann, Heiko. A History of East European Jews. Budapest; New York: Central European University Press, 2002