It’s likely to be a combination of things although broccoli a key factor..
Roast dinners contain many different foods and typically contain:
4-5 vegetables
At least 1 type of meat
Potatoes
Yorkshire puds
Gravy
This is getting on for 10 major food types which your body will work extremely hard to process and has a similar effect to “mixing your drinks”.
Name me 3 other dishes with as many ingredients in terms of major food types…
It’s like a mini buffet… and your body lets you know about it…
We should all be eating in a way for food to pass through the system easily, some foods are extremely taxing, chips for example.
Have fun, know your body and listen to it, we all found this site for a reason.