Hi Eric,
I am not sure the iRules is going to provide you that level of information simply because it doesn't have enough commands to provide you the specific reasons. However, if you know the end result you can possibly log that and assume that it choose "Primary, Secondary or fallback". For example you assume that if secondary was chosen it it was because the Primary became unavailable due to a Health-check issue or someone might have changed the logic. However, you might also want to look into iControl to see if that would provide you greater depth which I defer to other members of this forum that know it like the back of their hand.
Bhattman