Perhaps BYD view ADL bodies as rattle boxes and feel they can offer better themselves?
There is also the angle that BYD didn't want to continue to give piece of the pie to ADL. I suspect they always intended to establish themselves through the ADL partnership, and always planned to bring their next generation in without ADL involvement.
Arguably they've nothing to lose, if they struggle to sell "pure" BYDs and ADL simultaneously struggle to sell their own product, I'm sure they could revert to new version of the previous arrangement as by that point it would be of mutual benefit.
It's a bit Trojan horse approach but it's a tactic as old as time.
I have a little sympathy for ADL, but they've been selling at volume for quite a time now, but question marks over quality have continued and it appears they've not been profitable. Designing a new product is an expensive business though, is it just the outlay from that perhaps?
I feel what's gone over at Wright's smells extremely fishy politically, but irrespective of that they have re-emerged as a competitor, and, whilst reviews are mixed, there are suggestions that their build quality has improved (and as someone suffering 2017/2018 Wright's vehicles on a regular basis I really hope that is true!!)
Yutong's success probably wasn't factored in by anyone, but it seems completely predictable. They had the right combination of product, price and - via Pelican - service. The first operators loved them and word has spread since.