BSD doesn't allow relicensing. I don't think any license that does either. However, BSD allows the licensees to incorporate the source however they want as long as they keep the notice visible.It doesn't transfer the ownership so re-licensing isn't possible.

So basically Redis becomes a licensee of the third party contributors and BSD happens to be quite permissive that Redis can relicense their own parts while keep using other people's creations under BSD without limitations.