5 Soft bounces becomes a Hard bounce

If an email address gets 5 soft bounces in a row, Carma will convert the bounces to one hard bounce and will set the recipient as inactive.

This is primarily to avoid increased problems with deliverability which will occur when you keep on sending emails to recipients that only reports as soft bounced over and over again.
The main reason to this is that the receiving server is incorrectly categorizing a hard bounce as a soft bounce.