I was going to ask which browser but it doesn't matter. 'Settings' in any browser (that I've seen) lets you set your home page, even if solus packages goes to Solus at default...you can simply change it. You know all this.
So 2 theories:
1) solus is still linked as a 'favorite' or 'top visited' or 'speeddial' type thing ON your home page and wires get crossed when you open it? (unlikely)
2) if a browser setting is getting over-rided intermittently then something is telling it to over-ride. Don't know what that something would be, though.