rear tire locking

I'd like to ask you guys something about the brakes. what can cause to rear tire to be locked and slide easily? recently i was exercising to drift using the brake on dirt. few days ago when i was riding on the street i realized the rear tire was locked even i push the brake a little bit. it even happens when i am braking on traffic lights.

it feels like when I'm breaking at the same level for few seconds the brake locks the tire. so i don't think it's about the softness of the brake because it doesn't happen suddenly at the same point of brake.

bike is new and i put 3000 km on it so far. so it must not me about the pads.

I'm going to bring it to service to be checked but I'd like to know what can cause it before i do that. what do you think about it?

Sent from my MI 8 SE using Tapatalk

in case you wonder; i just took the brake off, cleaned every single part of it, sanpapered the pads, lubricated the parts needed and put it back. it works like a charm :) thanks god i was not convinced by the repair shops. i already knew how things work here in china but keep learning. "méi shì, méi gūan xi" these are the words that i really hate.

Sent from my MI 8 SE using Tapatalk