Why did my dad change all of a sudden?

I admit that my dad is a pretty cool dad (sometimes) but he is kinda hard to understand. Both of my parents can be strict at times, but they are ok for the most part. I'm 14 and I've been friend's with Jake for about 6 years (he is 15). Both of my parents really like him because he is a really nice guy who I really like. We've been together for about 8 months, and my parent's aren't exactly happy about it, but they don't really have anything against it. My parents allowed him to sleep over last week. But he had to sleep in the guest room with my dad (my dad didn't want him sleeping alone). That was the first time he ever slept over, and apparently the last. My boyfriend described my dad as having eyes on the back of his head. He woke up to go use the bathroom, and my dad fallowed him and waited for him in front of the door. And in the morning my dad was acting weird, he wouldn't take his eyes off of him. And now my dad isn't treating him the same. My dad used to be all friendly and stuff, but now he is just serious and not himself. I just don't understand, I mean, my dad used to like him but now he is just serious all the time and practically grilling him alive. Why did my dad change?