Ok so i decided to do a little testing of my own in regards to the spinners opening the bowling on PC. I had my HD die yesterday so lost all my career and DBC related files so i thought id see how this problem goes with default teams and then unedited "get best teams"
First of all started a career as an opener (2nd on batting order) with on-disc teams as Victoria. The issue WAS NOT present. At all. (played thru 2 FC games, batted first in one, second in another so batted in all four innings over the two games)
Immediately quit and downloaded Get Best. Started another career as opener (2nd) with Victoria against Queensland. Batted second. First thing i noticed was that James Hopes opened the bowling first. ( a horribly slow medium pacer) and sure enough the spinner opened from the other end.
The fact that Hopes also opened the bowling to me suggests that it could well be a ratings thing with User created teams. However im SURE this hasnt always been the case. As Blockerdave says, The first innings of the match always has what you'd consider normal opening bowlers, but then innings 2 3 and 4 have a spinner and often another mug opening. It really is very odd. Im about to check and see if it happens on PS3 as well. if it doesnt, it may suggest that whatever has caused this has been in one of the latest PC updates (obviously)