because your phone has low RAM. ... your operating system and your running apps already are using some of the memory, and when you open a game, there is no more memory left for the game to run and hence your phone must either shut down or restart to clear some RAM.
Software Glich hai koi jab mene one ui 1 new New aaya tha tab update kiya tha Note 9 me to same problem ho rhi thi Automatic Restart hota tha phone fir bad me 2 patch update se fix hogaya
Go to settings>battery ..you can see battery option,Performance mode... you will get heat and battery lasts quickly but performance will high no hang,no lag..
if the problem persists reset your device then install limited apps,then play pubg..
