I don't think inferring causality here is far fetched. They were fine with the content before. And we know it because said content was banned in some countries (including Australia) to comply with local regulations, so they clearly knew what the games were about.

Given that fact we have two options: either they decided to change their approach to content moderation and remove games that previously passed all their checks, with these games being coincidentally the same that were requested by Mastercard; or they decided to remove every game requested by Mastercard regardless of Steam's own policies.