Why does Clemency get interrupted? And why does Sword/Shield Oath reset combo?

[Reynhart]

To avoid a dps penalty via a combo break, PLD needs to anticipate a max of 3 cooldowns. If you don't bother with anticipating, you *might* take a dps hit from a combo break.

Ok, then let me explain.
By switching to Sword Oath and switching back to Shield Oath, you "waste" 2 GCD for 0 damage. So, in order to gain DPS from stance dancing, you need time to do at least 2 full combos before switching back.
Which means...oh, look, 8 GCDs.

And Defiance is still offering its most useful benefit even after you take a hit, by increasing cure received. It means that PLD is more penalized for being in the wrong stance when the hit occurs.