How did my maltipoo get a broken hip (pelvis)?

Yesterday I left for work at about 7:45am, and my boyfriend (who absolutely loves my dog) said he would take him out for me. We live on a 2nd story apartment Benji, my dog, was fine according to my boyfriend. He was running up and down the stairs like normal he even ran into his crate like normal. My boyfriend also had to be at work an hour after me so we were both gone all day. When I got home I could hear Benji whimpering in his crate, and normally he barks. Well I looked in his crate and he was laying down which is odd. I opened the door and he came out with his back left leg lifted. I started freaking out!!! Soon our roommate (whom is a male) came to see what was wrong... First thing I asked was "Since you were home all day did you let Benji out at ALL to go potty or play" he said NO WAY!! when normally he does.... Then I asked my boyfriend who had just walked in the door if he knew what happened and he was completely oblivious to what happened to my dog!! My boyfriend didn't do it because he was very sensitive to the fact that he was hurt. He was very attentive to Benji. My roommate likes to play with him to rough and I ask him to stop. Benji is only 6.25lbs!!! The vet said he did NOT break his hip in his crate, which I completely believe but maybe there is an underlying issue of bone density problems. I know it's a long shot, but he does have bowed legs in the front (they bow inward). So my question is...... Do you know of any maltipoos that might have low bone density or some sort of bone disorder?? Or what kind of force it takes for this to happen to a 6lb, 1 year old puppy!?! Yes he's had xrays. The bone was only broken once, it was not shattered. If that helps?! He's on antibiotics and pain medicine and bed rest for 8 weeks.