Answer to "Why does my phone sometimes just turns off itself?"Have you heard about the "piece of paper/card" trick? Some Fairphone 2 have batteries that are slightly too small and so any movement causes the battery to temporarily disconnet. So put a small (0.5cm) piece of paper at the bottom of the battery to keep it firmly in place.
Answer to "Why does my phone sometimes just turns off itself?"I have had this problem too. The phone seems to get stuck into thinking that the battery is very low, even though it isn't - hence it keeps shutting down. What seems to sort it out is removing the battery for 10 mins (i.e. longer time) and then replacing it and fully charging it.