It's for evacuation purposes, basically. Disabled guests have to be evacuated first, and it's easiest for them to be on the back row, which has the shortest journey down the lift hill (presuming this is where the train's stopped) back to the station. The only exception to this rule is if the train has stopped right at the top of the lift hill, part of it has gone over the crest, and it's still engaged on the chain. In this scenario, from what I remember, the front row would be evacuated first to reduce the weight of the train at that point, thereby reducing the risk of it accidentally disengaging from the chain and making its way around the layout. After this, the back row would then be evacuated. I might've got that a bit wrong somewhere, but I'm pretty sure that's why disabled guests are put on the back row.