I did a round of minor league recommendations in one of my leagues, and my Hi-A pitching level is randomly set to tandems. I have never seen this before at any level in any league, I am curious what triggered it

Are you short of actual DUR/STA combo SPs? One thing I've seen is that the AI doesn't seem to care how "good" the players are; it looks like it sets up your SP rotation based on who can get through more innings. Maybe it's seeing that your guys can't go, say 60 pitches every five days.

Similarly, if you've got a pitcher who's like 15/40, it'll automatically make him an RP.