According to bystanders the 11-year-old gelding by Orbis got in deep to the jump, left a leg behind, and somersaulted.
Reports indicate the horse may have broken his back in the fall.
Course builder and event director Alec Lochore described the fence to British newspaper Lynn News as a "very straightforward, kind, fence at the end of the course." He said it had been included in all Burnham events and had been jumped by thousands of horses in the past with no problems.
The jump was removed from the course after Porloe Alvin's fall. Wiegersma did not ride her other two horses after the accident. She also had a bad fall earlier in the week, at Somerley Park on March 30.
Porloe Alvin placed second at his previous start, the CCI two-star event at Barocca in Portugal, on March 14.