What causes the "ripples" that extend east-west along the mid-atlantic ridge? Below are two cropped screenshots from the super-cool video Draining Earth's oceans, revealing the two-thirds of Earth's surface we don't get to see with the oceans "drained" to 4000 and 5000 meters below sea level.

We've all read about the magnetic field reversals leaving alternating lines of residual magnetic field trapped in the crust that go in the north-south direction, but I was surprised to see these topographic ripples that extend in the east-west direction.

What caused these to form?

I've added some annotation to indicate the direction of the ripples I think I'm seeing.
These are transform faults that result from differential spreading rates of the middle ocean ridge. As such they are part of the plate boundary. Possible causes are different production rates of magma, thermal differences of the sea floor, differences of relative plate movement.

https://en.wikipedia.org/wiki/Transform_fault

https://en.wikipedia.org/wiki/Mid-ocean_ridge

Paywalled:
Dynamical Instability Produces Transform Faults at Mid-Ocean Ridges

And:
Wilson, 1965: A New Class of Faults and their Bearing on Continental Drift

Inactive transform faults extending beyond the spreading zone are sometimes generally named "fracture zone".

But transform faults can also reach all the way to the next subduction zone (e.g. Azores-Gibraltar fault zone) and be responsible for devastating earthquakes.