After reading the wikipedia- article about him: why did he have to end his career? I know it was mainly because of an injury, but which injury was it exactly?
And was he really having such a strong decline in '92, when he wasn't named to join the dream team?

I believe he tore his achilles.

Yup. Zeke completely tore the achilles and most basketball players that have that type of injury especially in their 30's don't come back especially a guard that needs his speed and quickness.