The short version of why you're getting fat - your energy intake is greater than your energy output.
Long version - your new diet is very high in fat which increases the probability that you'll gain weight because -
- Fats are energy dense so it's easy to eat more calories than intended without realising.
- Fat has a much lower thermic effect than carbs and protein so less energy gets wasted as heat than would be the case on a low fat diet.
- Your body preferentially stores dietary fats as adipose tissue instead of using them to meet immediate energy needs.
- Fat is converted to adipose tissue in a highly efficient process with the metabolic cost involved being very low at ~3% compared with ~24% for carbs.
- Unlike with carbohydrates, protein and alcohol your body doesn't react to dietary fats by immediately and proportianally stimulating their oxidation which increases the odds that they will be stored as adipose tissue.
I could go on but what I'm trying to get across is that if you eat a high fat diet it's highly likely that you'll get fat unless you take great care to control your calorie intake but even then there's a good chance you'll gain a few unwanted pounds.