If you delete all your posts, that means the rank you have, for example, Member in rank and you have 65 total posts, then you delete 35 posts, the remaining 30 posts will be your rank, you will go down to Jr. Member again.
Note that your rank depends on number of two-actvity periods in which you have been active too.
Assuming the remaining 30 posts all have been made in a single two-week actvity period, your activity will be 14 and you will be a newbie.
I saw some users before deleted some of their old posts even though they had posts more than the activity but still their accounts went ranked down.
How is that even possible, could that have been a glitch?
Yes. Visit the topic created by GazetaBitcoin.
The curious case of the Brand New user with posts