It could be a bad app, a virus, malware, etc. The only way to tell for sure would be to search the phone, going through all apps, then have the phone checked for viruses and then for malware. All of which can take a while. Or you have the option to back up all personal data to a google account, computer, etc. Files, contacts, music, etc. Just not apps. And then do a "Hard Reset" on the phone which will wipe anything and everything on the device and then return it to its original state when it was purchased. You can then put all the info back on the phone and redownload and install any apps.
Should you choose the option to "Hard Reset", the instructions are as follows...
NOTE: Doing this will clear 'all' the data on your phone, so it...