What causes this sidewall wear pattern??

It's just the carcass flexing and showing through the rubber, I've never even given it a second thought  🙂

The tyre squishing and wallowing because most people run low tyre pressures for grip. My wife goes through tyres faster than me because of this. She is a lot lighter than me and runs her tyres around 20psi where as I have always preferred 30ish. 

it’s just what tyre sidewalls do

My Vittorias do the same as well. Doesn't help that the grey sidewall is probably a very thin layer, added to the tyre, and ultimately wears off.

Nothing to worry about though. Just looks rubbish.

It's just standard wear. Are they with or against the direction of rotation? 

My understanding is it's the sidewall flexing, often while braking

Sidewall flex... Caused by low tyre pressures (often required to attain the desired levels of grip).

I think it’s flex and the tangental forces of braking and pedalling putting the tyre under tension

Ah fair enough, I do run relatively low pressures, usually 25ish psi, good to know it’s harmless as I do rate the Agarro highly as a fast but still grippy rear

I had a pair of tan wall Teravail Corronado’s eventually give up the ghost and not hold air with this but it was after a particularly long use at low pressures. Happy trade off as far as I’m concerned. 

Per others comments its sidewall flex. On lighter duty carcasses it can result in airloss. After a period of contemplation I took it as a sign that I should be running a tougher carcass, and I've not seen it since.

I had a pair of contis fail because of this fretting - eventually the bead came away from the carcass as the flexing had cause a fatigue failure - extreme use tho of a lightweight tyre on the tandem at lowish pressures 

Sidewall 'flex' when running lower pressures.