Thank you for that information. Generally dogs start sudden behavior changes for one of three reasons. The are frightened, they hurt themselves, or they have a health problem.
The fact that you are not aware of anything happening does not mean it did not. If you live in a rural area then there may have been something that went off there such as a gun or firework, or there may have been a particular animal sound that was new.
If he is out on his own at times then he may have hurt himself and associates an area or sound with that.
An example: A bee sting is something that dogs have difficulty with because not only would something like that hurt, but the dog then associates any type of buzzing noise with that pain.
If it were a health issue then it would be neurological in nature such as focal or aura seizures. Dogs that go through these have a sudden change in behavior.
That being said, behavior wise he is at the age where certain imprints are left with him. Dogs go though this throughout their lives into about the 5 year. Imprints change, a dog that was once fine with a noise or area, suddenly becomes frightened of it. This is a normal part of their life. At that point it is how the owner handles it that will determine how the dog spends the rest of its life.
The fact that he is calming down on his own is good, but you must make sure there is no soft talk or coddling as this is seen as form of reward and the behavior will get worse. If you can, when he begins to act like this work him in commands to redirect the behavior and then you can praise and reward that behavior.