Why are my dogs teeth falling out?

I adopted a stray dog that I found at work about 3 weeks ago. I took
him to the vet, and they gave me steroids and anti-biotics for his skin
infection for 2 weeks (this poor dog was in bad shape). He was 10
pounds. The vet looked at his teeth, and said they looked very good,
and he looked to be between 1-3 years old. I noticed he had what
appeard to be a cavity, but otherwise was fine.

Monday (which was 3 weeks to the day that I found him), my neice was
playing with him, and noticed he weas missing 2 teeth. I said, "No, he
can't be" I looked, and low and behold 2 bottom front teeth were gone.
THere was one beside the missing teeth that was the shape and size of a
toothpick, and 3 others were loose. His gums were swollen and red, and
receded. I called my vet, but they didn't have an appointment, so I
took him to another. They were having the dentist in the next day, but
didn't have an appointment available. They thought is was serious
enough and called the dentist to see if she would fit him in. She did,
and they kept him over night. She called the next day and asked his
history. I said how long we had him, and he was rarely out of our
sites, and did not chew on anything at all, except for himself, he had
very bad flea dermatitis. Told him he was on steroids and an abx. She
said that would not affect him the way he was. She thinks his former
owner kept him in a kennel and he tried to chew his way out over a
prolonged period of time. She ended up having to pull 5 front uppers
and lowers.

Here is my issue though. 3 weeks ago, his teeth were looked at by me
and a vet, and they were fine... so signs and symptoms. Seemingly in a
week, and almost over night (my neice noticed the "cavity" that I
noticed, and this was the night before we noticed the missing teeth. If
they were that bad then, you would think we would have seen it the
night before), his teeth started to do this. Also, I started kenneling
him that night because he was not potty trained, and he did not try and
chew his was out at all. He whined a little, like he had never been in
a kennel before. It has been 3 nights, and he has not tried to chew out
once.

Does anyone have any ideas what could have happened. Thanks.