How's this for a theory??, page-116

Industry super funds and other large institutional clients (e.g. insurance companies) have their own large investment teams, comparable to those of funds managers on the buy side and research analysts on the broker/research sell side.

Industry super funds either manage funds internally and would no doubt deploy long/short or outright shorting strategies based on risk/reward. That or they allocate mandates of money to fund managers that can do this for them. Additionally, they probably also participate in securities lending (enabling shorting) do subsidise their custody fees so your example is probably a bit ignorant, no offence intended.

I've worked for several buy side fund managers who've offer long only equities strategies, and their traders use multiple ways to effect large buy orders. They give the orders to brokers to manage according to their instructions (e.g. participate in 10-20% of daily liquidity), use their dark pools, algorithms and the likes of Liquidnet.

My point is, industry super funds are becoming large enough that they are becoming like fund managers in their own right. Unless they are holding themselves out to members and the market as being against shorting due to philosophical reasons, then they are probably participating as one of the main culprits.