Hi,
I think it's most likely the Avast Self-Defense module which restores Avast shields over time. Typically malicious software would make attempts to disable Avast shields, so they could run while the system is unprotected. Avast tries to counter that.
If you would like to use Avast only for scanning specific files on demand, then perhaps you should opt-in to use passive mode. This way you will give up all of the active protection that Avast provides though, so it would be recommended to have another AV Software running as primary (e.g. Windows Defender, or the Microsoft protection software available for Win7).
You can enable passive mode in Avast Settings -> General -> Toubleshooting.