When Brooklyn was 15 and still at Arsenal's academy, he was quoted in the media as saying "I just want to play football". Then Arsenal released him, that dream was over and his parents encouraged him into photography. Romeo and Cruz left Arsenal's academy soon after that.
I assume that photography was a real hobby for Brooklyn at the time, so a good idea to pursue it or to use it to motivate him through sixth form. Unfortunately, that included The Book, for which he was profoundly ridiculed and publicly humiliated - I have no idea why they thought that was a good idea. That part (the book) actually looks like pretty shitty parenting to me. Along the lines of "any publicity is good publicity" - you can do that with your own work but you shouldn't do it to your children, even if they want it!
Five years later... Romeo is 18 and leaves school and David... buys him a football club (he went straight into the reserves of David's club). Now I know David was in talks over a club in the MLS for years, and it just happened to work out in time for Romeo, but I remember thinking at the time, my god, what a punch in the gut for Brooklyn. Of course, there are probably good reasons why David didn't offer this path to both his sons: I assume Brooklyn was out of shape by then (I assume Romeo had at least continued playing at school) and maybe Romeo really was the better footballer (no evidence either way). And of course, Brooklyn didn't say anything at the time but I do wonder if this caused him to look back and notice all the little, unfair things that happen in any family with multiple children and his mind made a narrative out of it. At least, it wouldn't surprise me!
So, if Romeo did take up with an ex and / or if his parents did disrespect Nicola at the wedding in some way, then that was probably just the final straw in a long history of real and imagined slights. He just had enough and when he was in a position to quit, he quit.
(I also don't understand why people are so ready to blame a woman when a man dumps his family. There is plenty of evidence that some men are happy to cut contact with their own children for selfish reasons. Why would cutting contact with their parents be any different?)