This is approximately the section in the video titled "Memory controllers hate you" (https://www.youtube.com/watch?v=KKbgulTp3FE&t=1399s), combined with the following section.

The actual explanation starts a couple minutes later, around https://youtu.be/KKbgulTp3FE?t=1553. The short explanation is performance (essentially load balancing against multiple RAM banks for large sequential RAM accesses), combined with a security-via-obscurity layer of defense against rowhammer.