I think a lot of folks do stuff for attention. And I think celebrities of all stripes think their opinions matter more than they likely should because they are famous.
It's not surprising that some amount of celebrities would end up going for Trump. He wants to cut their taxes, which they are likely acutely aware costs them quite a bit of money, plus he tells successful people that they've earned what they've gotten and that all the credit is theirs. That is very likely to resonate with a number of celebrities, especially those in fields where they are likely to see themselves as "self-made" and not part of a large system that helped them succeed. And, based on real world examples, it also seems that Trump appeals to those celebrities who are on the downside of their careers and are perhaps no longer as relevant as they once were.
In that sense, it's not surprising that Le'Veon Bell might be a Trump supporter.