Beagle puppy unusual behavior?

I noticed a rather unusual behavior of my 7 month male beagle puppy today. While on our regular walks, he started sniffing around frantically on the grass as if he found something very interesting. During this time, he would not listen to anything I say and did not even respond to me telling him to come and get his treats. Then all of a sudden, he started baying loudly while sniffing the grass. He has never done that before. I am sure he was up to something, the question is what. At one time, he pulled me off guard and broke loose from the leash and started running towards other people's yard. While I was eventually able to get him back on track and we finished our walk, and he is peacefully sleeping on my bed now, what do you think brought on this wierd behavior. Any way to prevent this in the future. Thanks in advance for helping me bring up my dog.