It's not happend during install. Avast is installed on my computer for many years. This happen during the periodic updates.
My computer restarts automatically with a notice that my Avast has been updated and Secure Line has been installed automatically.
Not anything I've ever seen but, I use SecureLine.
I've reported your concern to Avast let's see if that gets you a reply.
SecureLine VPN is a paid product, so shouldn't install out of the blue.
However, as Bob mentions he hasn't seen anything like that an neither have I.
I use Avast Free and haven't seen this in the antivirus updates, but even in program updates you may be offered alternative products, so just dismissing the What's New UI Pop Up, could have underlying pre-selected options.
As far as I know you can get a 7 day free trial of the Avast SecureLine VPN (typically this is after running the Smart Scan), but you normally need to complete a payment form for after the 7 day trial. So I don't know what is going on here, hopefully Bob reporting it will find some answers.