That’s one view. The first mass conversions took place during the brutal pogroms of the 1390’s. Jews were given the choice of conversion or death and as many as 100,00 did convert.
After the violence died down, the Church had a problem. It had never espoused forced conversion yet had no way to undo the conversions. So, they looked the other way as many of these Jews remained among the Jewish community and lived as Jews. Others, however, were happy to now live as Catholics as it meant that all the laws restricting Jews no longer applied to them.
The Inquisition was established in 1478 to uncover New Christians still practicing as Jews but without much success. Torquemada, to justify his failures, said it because of the support that the Conversos were receiving from uncoverted Jews. So, he convinced Ferdinand and Isabella to expel the Jews.
Ultimately, that was the reason for the Expulsion.