If you delete all your posts, yes your activity will go back to zero.
Activity works by taking the number of posts you have made within a 2-week period (capping at 14) and adding it to your current activity, which starts at 0. These values are stored in a database and updated with a script that runs periodically every other Wednesday.
So mass-deleting tons of your posts will lower your activity and consequentially your rank on the next Activity update cycle.
Next question:
Why do members do that? I think everyone likes it when have a high rank
Two reasons why they do that.
Either they want to hide something strange on their account that's why they decide to delete majority of their post.
Or they don't want to see their number of post became more higher since they don't like other think that they are spamming this forum.