This has happened a few times now. We'll be at one of the local parks, half way through the walk - he's off lead sniffing and trotting about and all seems ok. He seems to suddenly get over excited, starts ripping up grass then starts lunging at us, jumping and snapping. Jaws open and leaping up to mid thigh, and it's hard enough to draw blood.
We manage to put him on his lead and hold him away from us at arms length, so he can't hurt us. He then calms down after a few minutes and is back to trotting about and investigating again.
Any ideas what could be causing this? We have tried distraction with a ball and treats with commands without success.