Some participants receiving "double-check this link" message

During a group meeting yesterday, one user received a message when he clicked on the meeting link that said:

Double-check this link

The link [link]...is taking you to another site: [site url]... Are you sure you want to continue?

It did take him into the meeting that I was hosting, and no one else received that message when joining the meeting. Any ideas why this popped up for just one user?