fishfingers15
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There were others at Kings Landing with Randyl Tarly, so it is not just one, though we have no idea the extent of the defections, there were at least some and that included the best. Randyl was just the focus of the discussion with Jaime, rather than have to show Jaime and/or Randyl talking the others into betraying the Tyrells, which would have been rather boring.
the series has already shown that the loyalty of the bannermen can be a very strong bond and can be breakable with the right pressure/reason to break the bond. It is not one of those things where everyone is ready to revolt at any second nor is it an unbreakable oath/bond.
It is politics isn't it.
Well, this is like, 'Oh this happened, perhaps the reason could be because of that'. The show should be doing 'This happened in episode X leading to Highgarden falling easily in Y'. Surely, Randyl Tarly siding with the Lannisters is hardly enough for that. To excuse this with the 'Highgarden was only known for it's beauty' is nonsensical. Perhaps you know it.