Does the pasture grass make horses hot?

I noticed whenever I let my horses in a grazing field with lots of grass before riding they get really hot and overexcited when I bring them back. I mean, they run like crazy in big circle around me till they are tired and sweaty (not good if you want to ride them of course). So now I don't let them graze until I'm done with riding or don't plan to ride. But I'm just curious if it's something abnormal...

They have nice hay and grass on "main field" - no as much as on "grazing fields", but still enough for them to pick.

I guess it would depend on what kind of grass is in the pasture. You don't have any wacky weed, do you?

Seriously, I haven't had that problem with my horses.

Oh, no, I just buy pasture mix from my local horse store. Nothing special or weird. I'm guessing may be it has too much sugar in it, so that what makes them excited.