Oats are a hot feed which means they give the horse energy or make them hot.
Take the oats out! Every horse I have ever had has gone abit mad when they have oats in their diets for these exact reasons ^
Like Sierra said, oats give energy! Some riders just give oats to horses when they are competing so they have that extra energy!
Yes, oats is a natural energy source there for it does give your horse extra energy