Saddle burn question

I have a question. For some reason, our 8 year old gelding Harley developed what I have been told are saddle burns. They are on both sides, behind and below the withers, about where the base of the western saddle tree rests. They are areas about the size of my hand where the hair has turned white (he's a buckskin) but otherwise there is no apparent injury.

This has just happened in the past week. The strange thing is that we haven't changed anything about the saddle nor saddle pad. We've been riding him with the same saddle and pad for the past year. His workouts have not even been as long or hard as usual in this past week. The only difference I can think of is that we have been really working hard on keeping his head tucked and rear end gathered under him this week in preparation for a show a week from Saturday. We've also been lunging him for warm up in the round pen with the reins tied to the saddle horn tight enough to keep his head tucked vertical. Could that have changed the pressure under the saddle enough to cause the saddle burns (if that's what they are)?

Can anyone suggest something we can do, other than not tucking/rounding his body? Do I need to add a second saddle pad, even though the one we're using is pretty thick? Is there something I should put on it? My mom said she doesn't think it will ever go away now that the hairs have turned white, but that's hard to believe since there didn't appear to be any prior warning that anything was wrong. :o(