TA position in different discipline?
Some departments need a lot of TAs. Math, for example, since there are a lot of people taking elementary courses. If there aren't enough graduate students in the department to cover the need they might need to look elsewhere, but probably within the same institution.

But this would vary a lot between institutions. I don't find it to be odd, but neither do I think it is common. If you are qualified to assist, or even teach, an elementary course then the position might be open to you. Note that it is probably also true that some departments have unmet financial needs for graduate students and this gives them a way to provide support.