My question is:
1] How could this happen
It could happen as a result of the account holder deleting his or her post history gradually which affect other aspect of the account prominence.
2] Does he delete all posts and activity will go back to the beginning?
If the post is been deleted, it automatically affects the activity when this is done, the activity gradually reduces as the post is being deleted. It also affects the rank too because the activity is needed for rank up. So therefore a drop in activity also causes a drop in rank.
3] If I'm not mistaken, he should at least be a hero member
How do we know when you did not drop the link here for us to see for our self because what you have posted here might portray a distorted information. S we can not tell exactly the rank of the account
4] Is this normal? or am I the one who doesn't know about phenomena like this?
How is it normal mate. Does it looks alright to you for some to just decides to start deleting his or her post history that took them precious time and commitment to build up to that level. Do you know what that means.
You could also be having this thought if the account holder is trying to cover up something they have done in the past