It's like inflammation. There is an endless list of causes, but the outcome is more or less the same... and it's never pretty.
Why do dogs lag? In my experience, it basically comes down to a few main reasons: the dog is bored, the dog is stressed, or the dog is confused (aka inadequately trained). So, how do we tackle lagging? In most instances, it's a matter of training, so this is how I tackle the problem: