Interesting, I wonder what the triggers could be.
Others seem to be indicating OD is often dropped on guards. For my 35 year old big, it was 1 IS drop and 2 RB drops this season, although he did drop in IS once last season.
So possibly it's OD and PA are more likely to drop for guards, and IS and RB for bigs? Very small sample size though, so it could still be completely random. Just when the "trigger" happens, one or two skills get hit hard.