How did I gain weight?

A few years ago I weighed 210 pounds. At 5’9” that gave me a BMI of over 30. Technically, I was obese. I decided enough was enough and went on a diet. I cut calories to 1500 a day. I weighed my food. I cut sugar and regular soda. I switched to splenda for coffee and started drinking diet drinks. I was successful. After 6 months I lost 50 pounds. I went from 210 to 160. I have been able to keep it off and I feel great.

My question is, what caused me to gain the weight in the first place? I have gone back to my old eating habits and do not gain anything. I still drink diet soda/drinks though. No sugared or HFCS drinks.

Was it the HFCS in the soda that made me fat? I’m beginning to think it was. What do you all think?