ZTE Blade A54 reboots and resets its Phone, Contacts and Messages applications to the Google's ones

ZTE Blade A54 phone comes out of the box with Google applications for Phone, Contacts and Messages. Yesterday I downloaded alternative applications from Fossify and set them as default ones. Now I heared the phone vibrating. It seems that it rebooted without any obvious reason. After it booted, I found out that Phone, Contacts and Messages applications were resetted back to the Google's ones. I thought that it was an automatic system update, but according to the log, the last system update was installed a month ago. How to find out which application / service triggered the restoration? Was it Google Play? Was it some system guard service? Howwto find it out?