Several members of my alliance, myself included, have had troops removed from our aw defense teams. Sometimes we catch it in prep, sometimes we do not. We thought it was from changing heroes but it has happened without the team being modified at all. Its usually one troop, sometimes completely deleted down to villagers, sometimes replaced with a less powerful version (i.e. 3* instead of 4*).
This can happen when you level up troops and accidentally feed one troop to the other. The feeder (the troop you have assigned to your defense AW team) then disappears and is replaced with a basic *.