I guess you can kinda forget or have a rush as a writer that makes you do certain mistakes while writing a long story. But this is unexplainable.

They were writing a story where the Boltons were struggling and trying spells to claim the north with no opposition since they lost their only connection with the Starks, when they were the 2nd strongest house before the Starks.

The Tyrells had it even harder, even with Gardener  blood, since they were stewards, they still had to pull out politic moves that won them the reach after DECADES.

The whole story of GOT is mainly based on just A RUMOR of bastards taking the iron the throne . 

And in the same timeline, it seemed logical for them to have a group of undoubtedly bastard daughters wiping out the Martells, after swearing loyalty to them, with their sworn soldiers watching and no single opposition from any other Dornish House to follow them. No civil war, no blood ties, nothing. The closest thing to resistance was the Maester trying to run.

Considering the writers make meetings before agreeing how they are going to handle a story, this could not have been a mistake and I am sure they talked about how senseless that was and that there were thousands of better ways to handle it. 

I am also curious about how this kind of contracts work, if I were investing millions in producing a series, you are going to give me a decent story whether you have thousands of projects coming up or not, or I am suing your ass and ruin your reputation as a writer. How did HBO let these things happen?

What were the writers thinking when doing this?