There are two potential reasons that someone might lose weight on a reverse diet:
You’re just coming off a low-calorie diet.
In this case, it may take several weeks of incremental calorie increases to bring you out of your calorie deficit. Until your reach this point, you’ll be burning more calories than you’re eating, which will result in weight loss.
Your metabolism is increasing faster than your calorie intake.
When you eat more, the stimulus of extra calories may cause thyroid and testosterone hormone levels to rise, as well as sympathetic nervous system activity to increase. In addition, NEAT (non-exercise activity thermogenesis) may also increase since you’ll have more energy to fidget and move around. All of this can cause your metabolism to speed up, allowing you to burn off all of the extra calories you’re eating each week, plus some. This erases the calorie surplus you’d normally receive through reverse dieting, putting you back into a calorie deficit where you lose weight.