It's because your starting PG has a 65 stamina. So Combs was needed to play PG most of the time. You only have 4 guards, one starter with 65 stamina, your 4th guard getting 16 minutes is reasonable.
Also, you say that Combs never played SG, that is not true. They only show the lineup at a few points of the game. But, look at the 15:47 mark of the first half (in the game against MSUB that you are talking about). Barnes came into the game and Combs stayed in, which means he switched over from PG to SG at that time. I am sure there are many other times like that