This was unexpected for the simple reason that it is not supposed to happen by definition.

As a reminder, ever since the financial crisis, in order to push the effective fed funds rate above zero at a time of trillions in excess reserves, the Fed was compelled to create a corridor system for the fed funds rate which was bound on the bottom and top by two specific rates controlled by the Federal Reserve: the corridor "floor" was the overnight reverse repurchase rate (ON-RRP) which usually coincides with the lower bound of the fed funds rate, while on top, the effective fed funds rate is bound by the rate the Fed pays on Excess Reserves (IOER), i.e., the corridor "ceiling."

Or at least that's the theory. In practice, the effective FF tends to occasionally diverge from this corridor, and when it does, it prompts fears that the Fed is losing control over the most important instrument available to it: the price of money, which is set via the fed funds rate.