There are 4 basic reason to find out what really had happened to your website.
It could be due to the site is Hacked
A redirect could have been added from your Control Panel
There could be a reason that a Plugin is Malfunctioning
It could be that you have mistakenly added another URL in the WordPress Settings section.
Remove website redirect virus with Rkill security software.
STEP 1: Use Rkill to terminate suspicious programs
STEP 2: Use Malwarebytes to remove Trojans, Worms, or other Malware
STEP 3: Use HitmanPro to Scan for Malware and Unwanted Programs
STEP 4: Use Zemana AntiMalware Portable to remove Browser Hijackers