Teenage daughter has extreme decay after 5 months, how?

We went to the dentist in November and they took X-rays and did a routine checkup and the dentist only claimed she has sensitive gums and that she should get better at brushing her teeth. She’s never had a cavity, so I was glad to hear even knowing her hygiene isn’t great that she could fix the gums issue by brushing more often with a special toothpaste.

Fast forward 5 months later and she comes to me and tells me she wants to stop eating sugar and I ask her why and she shows me two of her teeth having major decay where you can see the teeth are rotting away. I booked a dentist appointment right away and we went today.

We get there and they find that all 6 of her top front teeth are showing decay, where two of them are so extreme that it looks like it’s been years of bad hygiene. Neither I or the dentist could see how only sugar and bag brushing habits could be the cause, so I am concerned it could be other issues behind this.

What other things could cause a 5 month decay to this degree? I have pictures. Maybe I should post them for help.

What photos are there?