My first post here.
A320 has a very bad reputation on FBW system and stall characteristics on altitudes above 35.000 feet. Stall margin is reduced and even the bank of 25 degrees in HDG mode is very dangerous and can induce a stall. Judging by the last radar picture of aircraft position, pilot has deviated 120 degrees of course very abruptly, asking to climb to 38.000 feet, and that is NOT the way you do it in Equatorial region with the tops of Cb in that area that are very high and also when you have a squall line you should plan well before 20Nm from the storm. In this case he was literally running away.
Stall margin is VERY close for A320 on that altitude and with that kind of maneuvering and weather. Also, the last data says that crew added 800kg (because of the weather) of additional fuel and that makes them even heavier and reduces stall margin even more. Captain has only 6000 hrs.