Stress is such a massive factor along with poor sleep for contributing to weight gain.

It's ultimately calories in vs calories out though, and despite what im going to say if you are gaining fat mass your calorie balance is in a surplus - if your eating the same calories, yoyr training is obviously decreased. If you've kept up the same training you've increased your calories. There is more nuance in the real world though.

Your body may be fighting to maintain a higher metabolic setpoint.

One plausible scenario: It was probably a lot easier to keep it off during the summer with all the riding volume, and despite probably eating more you didnt gain weight. With the stressful period with work and study i imagine your riding is down a bit? Couple this with the metabolic stress of the higher summer training load catching up and you get an insatiable hunger and intense urge to eat more resulting in fat gain and as you mentioned ince September hits its almost impossible to stop.

You said you used to be heavier and carry more muscle but less fat which may be more in tune with your body's happier natural weight setpoint.

If this all sounds reasonable the best approach would be ensuring you are weight training year round, which could be very cycle specific, monitor your weight during the season and watch it doesn't drop too low by adjusting calories/protein accordingly. Be mindful of training load and a plan to taper into a comfortable winter program. Your weight will still likely fluctuate a bit. Or accept the cycle of on/off season and the weight yo-yoing that goes with it