There's no really good reason to stop stacking Expertise after 7.5%.
It's not really a cap as tank, especially as war tank.
While you're off-tanking, the useful defensive stats are stamina and strength (since SBar vaguely scales with strength via the pa buff).
While you're off-tanking, the useful offensive stats are strength, crit and haste.
You have to know that, on most fights, Hit/Expertise are NOT our best defensive stats.
There's two reason why most war tanks go for Hit/Expertise:
1) You want you rage generation to be as reliable as possible.
2) You want to increase your dps as long as it doesn't imply that the boss will kill you.
So, yeah, 7.5% Expertise is a cap as long as dps is concerned. Above 7.5% Expertise, you sacrifice as much survival for less dps.
But the difference isn't really that big and I don't really understand why war tanks focus so much on this "softcap".
P.S.: Note that Hit/Exp are not necessarily the best stat neither to maximize your rage generation nor have the more reliable rage generation (again, depends on the fight). But it prevents the bad scenario "The boss did parry/dodge three Shield Slam in a row, I have no rage.
" that is likely to kill you.