The Democrats voting for Santorum are big government big labor Democrats that are dissatisfied with Obama.
I have friends in a similar situation. Very conservative, esp. on social issues (staunchly pro-life, etc.) but work in a union job because it’s pretty much a must in their area and field.
It’s been my experience that there are a lot of union members who are good, hardworking folks. It’s just that most union leaderships are happy to protect the ones that aren’t as long as they cough up their dues.