TUI were slightly unlucky , as they had two flights scheduled to land within 15 minutes of each other, which coincided with a pulse of bad weather, hence the diversions
You've answered your own question, it's all down to timing. If you arrive and the weather is below limits then you hold and when you get to the point where you need to divert, you divert. I suggest the TUI aircraft would have all loved to have arrived, turned round and gone back home, unfortunately the conditions didn't suit that at their particular arrival times.
Sorry, but there's no conspiracy.........