This happened at the weekend. Simple stall or something more than that?

to me it looks like something to do with the aileron servo . it might have gotten locked

Too steep and sudden climb rate and induced simple stall with pilot responding very late to the attitude of the aircraft.

It's seem me pilot error, abrupt use of controls

Anyone even attempting to fly a jet is unlikely to be a low time pilot.
(No evidence of abrupt use of controls etc)
To me it looks like the angle of attack momentarily exceeded the stall angle.
Maybe a little tail heavy, maybe a little underpowered, maybe rotated a bit early.
Could possibly have benefitted from a little right rudder and a little forward stick.
Right aileron applied (about halfway through the left turn) may have aggrevated the situation by increasing the AOA of the port wing.

(Presuming to comment, despite no jet experience! But my guess - a bit tail heavy.)

Isn't that a short take off length for a jet on a grass field? And rather high alpha on climb

It is a sign of early take off

Looks like there was not enough speed causing the stall. Probably the take-off did not build up enough speed, assuming the power was adequate. I am ruling out the possibility of an unexpected flame-out here: if that happened shortly after take off....

But the turbine sound shows that this probably did not happen

that abrupt use of control stick I even do not seen any rc jet but may be the angle of attack increases so much (use of pitch stick) that there was sure crash no use of rudder or aileron can make recovery that was too late . As sanjay sir said it was early take off attempt "not enough speed" it is under power take off decision.

all lost of lift under side of the wing


There are "canard" used in su 30 mki that makes the fighter nose pitch down when ever u see su take off, that's something new tec. Used in tail heavy highly aerobatic fighter airplanes
Sorry I go little bit out of topic.

first- vry short take off!!
second -there was no need for a roll just after take off
final- quick realization could have saved the plane.

I think there was no planned roll. It lacked speed, and tip stalled

yup!! totally agreed with sanjay sir!!