I understand what you are seeing, but perhaps I can give you some figurative language that might help.
Think of aging as your body's response to the increasingly heavy weight of time weighing on your body. When you get something that freezes you in time--a Denarian coin, becoming a Rampire--then all of the additional weight keeps piling up, but it's prevented from actually affecting the body by this power. When the power is gone, all of that weight falls on them, instantly aging them as much as if they had not had the power. Or, if you assume accelerated aging like in the DV rather than instant, what was keeping the weight off is no longer supported, so the weight starts falling through, more and more of it piling on the person, until they die.
Now, that's obviously not a comprehensive theory, given that organisms grow into their prime before they begin weakening, but it might be illustrative to think of it in that manner.