According to the news article, this is what happened:
The string of events started shortly after noon when FedEx flight 881 approached for a landing on the center runway."In all three of the operational errors, we had more than 95 percent of the required separation. But our job is not to have loss of any," said Peter Suflaw, head of the air-traffic control union at the tower.
While still in the air, the pilot reported an auto throttle problem to the tower, which required the plane to slow faster than usual.
When that happened, the plane coming in behind it, a regional jet operated by Pinnacle Airlines, broke the 5 miles of separation the FAA mandates between heavy and small jets.
The FedEx pilot aborted the landing and flew around to try again. As it approached the runway for the second time, the separation between it and another FedEx plane at an adjacent runway was compromised.
"The diagonal separation between those planes should have been 1.5 miles. But because flight 881 again reduced its speed, the separation was reduced to 1.38 miles," [FAA spokeswoman Kathleen] Bergen said.
As both FedEx planes were preparing to land, another regional jet operated by Pinnacle was approaching for landing.
Its 5-mile separation was reduced to 4.86 miles.