This was considered unusual because the surface currents of the Pacific Ocean flow westwards at the equator, following the direction of the winds.
In 1952 Townsend Cromwell led a research party to investigate how the currents of the ocean varied as a function of depth.
They discovered a fast-flowing current that flowed eastwards in the deep surface layers.
The cause of the pressure gradient is at least partially the trade winds blowing from the east westward.
In non-El Niño years, the Cromwell Current is forced to the surface by underwater seamounts near the Galapagos islands (this is called upwelling.)
Upwelling, however, is a sporadic phenomenon; it fails to occur on a regular basis, and so the food supply comes and goes.