it is not hydroplaning...
whatever be the hydroplaning speed.. for the boeing 737NG it will be above 60 kts.. even if it were bald tyres..
the runway excursion happened..in the taxying phase of the flight.. and at the end of the runway when the pilot was turning around for lineup....there is a boeing limitation of not to exceed 7 knots speed for turns and if the surface condition is seen experienced undulating/ rough or slippery one could slow down upto 2 kts....worst case stop and ask for the tow truck..
pure act of indiscipline and so the aircraft went off.. it is a long runway at dhaka.. no reason to go to the edge for lineup and show-off.. this was a very stupid and irresponsible piloting act...