Hot Horses in the Spring

Why is it that every spring, the horses get too hot for their britches?  Every April, out comes my western shank bit for Sheylan - I normally ride her bitless, but even in the spring a regular snaffle won't suffice when she's out on trail and makes a mad dash out of pure glee.  My jumping horse, Eddie, who performs like an angel all winter in the indoor arena, needs draw reins added to keep him from galloping at every jump. I don't know whether it's the warmer weather, the softer footing, the after affect of the rider glad they can finally ride outside and not freeze that makes them act a little bit crazier. But every spring, all I know is that I blame all craziness on the weather.