Regulatory or perceived regulatory issues related to IRA's getting implied financing through futures contracts, UBTI reporting, or other compliance issues. Obviously they previously thought it was fine since they allowed it before. The regulators haven't been getting more lax lately and their favorite means of regulating the retail crowd is to push the bad news out via their brokers by requiring onerous record keeping requirements.
If true why the lack of notification and why are they implementing it haphazardly (some IRAs currently have the restriction, and some don't). This whole thing is very strange. Say it ain't so IB....why would you push all those accounts to other brokers?