you asking the same questions that have been answered, there was either fog or a storm going on , which is why the didn't see them. They didn't expect to be ambushed on the way to dorne.
I speculate Lady Olenna told Euron to fukk Tyrion over or maybe she doesn't want to bend the knee to the Dragon Queen, since she's old enough to remember what her father did.
Someone snitched ...
IDK they are suppose to be great navigators surely they can spot a monster ship approaching through fog. It would make no sense for Olena to turn on Danny right now with Cersei still alive. My guess is they caught theon and yahrah slipping since they were leaving Kings Landing while the others were heading there