This is a very unlikely scenario, but heres my thought.
So all these ess bots got banned. Now… All the other people who bot ess were like 'omg i gotta stop before i get banned too!' so they sell all their ess.
Now this is where it gets tricky.
All the runecrafters who buy ess from the ge are like 'eh bots will make the price lower and stuff, so ill keep buying under mid' (they had thought this before the bots got banned)
so there are tons and tons of offers in at ge for far below mid.
All the botters who are now dumping all their ess at mid are like 'omg itz n0t seeling!'
So then they lower their sell offers. and then again they're like 'oh mai gawd its still not selling! i better sell this at super blow mid so i can get paid before ess is worthless'
so they sell super below mid. At the same time all these runecrafters and other people who are buying ess below mid are like 'oh snap son! i got this ess for like 300% below mid! i better buy moar moar moar and stock up before the prices go back up'
so they buy for super below mid.
now a day or 2 later…
the botters who are just selling their ess now are like 'omfg its not selling and the price is taking a nose dive! i better sell at 1geepeez each so i make like 500k'
so they sell. and then all the people area already buying below mid.
and this cycle repeats until it drops. a lot.
This probably is very

, but weirder things have happened.
also contributing to this as riwu has stated, lower demand for runes as rune cost in spells have gone down since eoc.
note: that little image is a Farfetch'd, its a pokemon.