I'm sorry, I have no further information. I did not install anything new recently, and I almost always (I hesitate to say never) use manual installation.
I've had Avast installed for years, never a problem.
What I can conclusively say though is that Avast Secure Browser was installed on March 14th (most likely _finished_ installing yesterday as I hadn't booted my computer in over a week) and I did not explicitly install it. I most certainly would never have agreed to it being the default browser nor of auto-starting on Windows login.
As I said, my best guess is that Avast updated itself last week (as it periodically does) and as part of that update it included the Secure Browser. Please note that it was an update, not a new install or reinstall. As such it should not include completely new components, or, if it does, that those should be clearly flagged to the user. As a user I expect a standard antivirus software update to be an updated engine and virus definitions, no more.
Anyway, I'm sorry to say, but for me, Avast has just taken a 95% hit to its credibility and trustworthiness. Although I would be interested to hear from other users whether something similar has happened to them or whether my situation is a once off. If the latter I might reconsider my impression.