Good question, didn't notice that.
I have checked my album and came up with the following:
year
subject
date location
1989
Loon
reverse
1992
Parliament
obverse
1994
War memorial
obverse
1995
Loon
reverse
1995
Peacekeeping Memorial
obverse
1997
10th dollar loon
reverse
I am still missing this one....
2004
Loon
reverse
2004
Lucky Loony
reverse
2005
Terry Fox
obverse
2006
Lucky Loony
reverse
So, on the normal circulation coins, they're on the reverse, on the commems it varies.
Can't find a "system"