Why am I registered for income tax

Hi I was working last year earning R3000 a month. I got an sms this year stating that I have been registered for tax accompanied by a tax number. I want to know why this happened?

All South Africans who earn an income even if it is under the tax payable threshold is liable to be registered and have a tax number.
Your previous employer needs to declare the income they pay you to SARS and needs a tax number on the declaration that's probably why they registered you for tax.