Website hijacked on all hyperlinks

All things updated to their latest.

Whichever page is loaded on any browser and device, the first click of any link (on page or inside pull-down menu) would yield a new tab on browser to a random website. It is very annoying and making the business site spam-liked.

We installed a couple anti-hack plugins to scan all files in the domain but this hijacking is still happening.

Any suggestion?