I recently switched to EpicSpawners, where I came to the conclusion that spawners with a chance of spawning jockies are bugged.

For example, I have a pigman spawner which spawns more chickens then pigman's. The same happens for zombie and spider spawners ect. The chickens do however attack the player.

There is a discussion going on here which provides more information: 

https://www.spigotmc.org/threads/zombie-spawners-spawning-chickens.338686/