Strange behavior

My 4 year old black lab Buddy has started some unusual behavior, normally when he is let into the back yard to do his business he would let out a bark when he is done, now he sits in the middle of the yard as if he is cowering and will not come in on his own, i have to go out and physically bring him into the house but only during the day at night he will come in on his own. Need help figuring this out.

[lindapalm]

Is he seeing something during the day that is scaring him that isn't there or he can't see at night? Something in the pathway to your house, or a noise in your house during the day he doesn't like?  Definately strange.