It is probably just that, the joint stiffened and needed popped. Unfortunately because they are always standing on their feet, and the added weight, it doesn't feel like popping your knuckles. Think of it more of like if you were kneeling and went to get up but pressed down on the leg that was still under you to get up and it popped against the ground. It is very possible that this is simply because of cold weather. With my horse and the horses I use to take care of I tap on their leg and verbally ask them to lift and when they move their foot I will pull up until they pick it all the way up and pop it on their own if it needs done. Always stretch before riding, especially if the joints are popping. My uncles horses is 28 and has severe arthritis, to the point of not lifting his legs if he doesn't have to. I got him to pick up his leg so i could stretch him and his joints popped and the next time I saw him he was running full out through the hay field...lol
It all depends on Medical History, age, diet, temperature. Keep an eye and if it keeps happening maybe try a joint supplement.
~Set Your Heart Free on the Wings of a Horse~