What's the rules say on this? Ball was definitely lodged in there but easily retrievable.
It actually doesn't matter how retrievable it is. You're looking at 6.09 e & f:
6.09
The batter becomes a runner when --
(e) A fair ball, after touching the ground, bounds into the stands, or passes through, over or under a fence, or through or under a scoreboard, or through or under shrubbery, or vines on the fence, in which case the batter and the runners shall be entitled to advance two bases;
(f) Any fair ball which, either before or after touching the ground, passes through or under a fence, or through or under a scoreboard, or through any opening in the fence or scoreboard, or through or under shrubbery, or vines on the fence, or which sticks in a fence or scoreboard, in which case the batter and the runners shall be entitled to two bases;
And 7.09(f) Two bases, if a fair ball bounces or is deflected into the stands outside the first or third base foul lines; or if it goes through or under a field fence, or through or under a scoreboard, or through or under shrubbery or vines on the fence; or if it sticks in such fence, scoreboard, shrubbery or vines;
Once it goes under there, it's a ground rule double. Upton made it more difficult on himself by actually retrieving it after throwing his hands up - should have always left it there.
Technically, it's called an automatic double, but people have been calling them ground rule doubles for a long time, though technically those only apply to special rules for that particular ground. They probably should not have reviewed it under the technical rule of ground rule double, but nobody understands the automatic double rule vs ground rule double rule so it's moot.
The call should have been double from the moment the ball went under the fence and Upton threw his hands up.
