I seen some hacked WordPress sites where redirects are added to the pages. Some of those may be only happening when user comes from specific site (referrer based). But it also may be some sort of misconfiguration of network, etc. Unfortunately it may be a bit too technical to troubleshoot, so consider getting someone experienced to check it.
So, I would start with inspection of the .htaccess files, page HTML code (look for injected meta tags or javascripts). Then need to check scripts. Run a scan with Sucuri, inspect theme files manually.
So, I would start with inspection of the .htaccess files, page HTML code (look for injected meta tags or javascripts). Then need to check scripts. Run a scan with Sucuri, inspect theme files manually.