From Apache's website:
Signal: USR1 (SIGUSR1) is a graceful restart. It causes the parent process to advise the children to exit after their current request (or to exit immediately if they're not serving anything). The parent re-reads its configuration files and re-opens its log files. As each child dies off the parent replaces it with a child from the new generation of the configuration, which begins serving new requests immediately.
Check the logrotate file of Apache: /etc/logrotate.d/apache2
It may be possible that logrotate settings are causing the graceful restart of Apache. But I do not think it should a concern for you because Apache will not drop any user request because of log rotation. You can change the frequency of log rotation from daily to weekly in order to prevent graceful restart happening so often.
