The most obvious thing to note is that your horse may simply be consuming more grass and consequently has more energy. Fast growing grass does not necessarily have a higher sugar content: in fact as a percentage it might actually be lower than when the grass is 'stressed' such as during a drought. However the sheer volume of consumption may over-ride this. Eating too much 'rich' grass is just like any other starch overload in the horse: the digestive system is overwhelmed leading to excess fermentation and toxic by products. This digestive disturbance may well cause behavioural problems in some horses, and of course is a pre-cursor to the dreaded laminitis in susceptible horses and ponies (article on digestive supplements here)
Secondly, spring grass is often deficient in Magnesium. Magnesium plays an important role in the functioning of the nervous and muscular systems, so a deficiency could cause behavioural as well as physical issues. This is why many calmers are based on Magnesium. Supplementing with Magnesium is only likely to help if there is a deficiency, but if your horse does display nervous or excitable behaviour connected with spring grass, it's definitely worth trying.
Finally, spring grass may have higher levels of oestrogens, especially if it contains certain types of clover. This can disrupt the horse's hormonal balance. Some mares will run milk at this time of year, and I've known a few geldings get a bit fruity!
So, if your horses is a bit full of itself at the moment, yes, it may well be the spring grass!