There is a rare genetic mutation that creates a zebra without stripes. It’s rare because they don’t often survive. When the lion attacks the herd, the zebras all scatter and run everywhere, in different directions. Because of their camouflage, the lion cannot tell one zebra from another, therefore cannot focus on a single target, and eventually becomes tired out.
With the stripeless zebra, the lion can focus on one individual. This is easier for the lion to hunt.