It's partly to do with natural root secretions and partly to do with the change in NPK levels mate.
Veg has high N low pk. Bloom has low N high PK.
If you have a look at some different brands of nutes, you'll notice that quite a few have started calling their pH down "pH veg" and pH up "pH bloom" wouldn't be doing that if it wasn't supposed to fall in bloom.
Otherwise you'd be feeding at 6.5, it would straight away be rising out of range and you'd only have pH up to fix it with. Same with if you started at the lower end. Wouldn't be able to bring it back down cos again the bloom adjuster only makes it rise.
It's pretty hard to see this in Coco as you can't measure things fantastically accurately once they're in the media but if you put the Coco on a recirc pump with a rervoir you can see it no problem on the meters.
Also, most feed charts tell you to feed waaay more than you need. To strong a nute mix will overpower the natural processes and make the pH swing go up in bloom.
So yes, if you follow a chart that says the pH will rise in bloom then it most likely will but it's because the chart is saying to feed too strong.
Again, you can only really see it when you use a reservoir of some sort but I promise you it's there. We can see the ppm levels go through the roof.