I think it's just a genetics thing, people store fat more in different areas.
One of the problems with rapid weight loss is that on a big calorie deficit you get a lot of lean muscle loss as well as fat loss, in fact a very high percentage of the weight coming off can be muscle. In order to counter that you ideally need to put some muscle back on, you need to concentrate on upper body stuff.
While you've lost a lot of weight you've changed your relative body composition, so you have a high fat to lean body mass ratio, you need to working at shifting that ratio so what you're left with is mostly lead body mass and not fat.
I kind of looked into all this before starting my 60-70lb weight loss, I started doing weights from day 1, I just do pressups and bought some cheap dumbells and do them every few days, you start gaining muscle surprisingly fast.
What you'll find is that you gain weight when you put muscle on, it's more dense than fat so you don't need to put much on before you see a weight increase. Keep a good diet with an overall cal deficit to counter this, you should try and keep a high protine diet as that will help muscle growth. You'll find that as muscle goes on your body has no choice to burn energy from fat and the fat goes down, at this stage I believe a lot of people have more success with losing the fat with exercise rather than with dieting.
It starts off hard, I could do like 2 pushups when I started at 250lbs and not lift much on the dumbells, now I can do about 15 pushups and have all of the weights loaded on to the dumbells. Weight training is great because it's so easy, it literally takes a few minutes to do initially. Just make sure you educate yourself a bit on weight training, rest every other day to allow muscles to recover, prepare to ache like a motherfather the first few weeks.
You'll like it after a while, I'm in no way vain but I do like seeing the progress on my "guns", chicks are going to dig it as well
