Am sure you understand that, these ranks are automated and works based on the principle of having a defined amount in merits and activity such that, they arr complementary to get ranked up.
Since posts constitute to that activity meter count, having them all wiped out will as well wipe out your activity and with nothing to complement the merit with, you can as well be demoted to a rank that suits your qualification by the forum.

That some way to state the fact. I'm sure OP understands but, if I may;
Is it only posts that counts towards your being active and counts in the activity meter?
I thought sending pm did matter too and counts towards have been active.