Worn seals,hoses, o-rings and crimped lines and steering components such as steering racks or gears they get hard and crack with both age and heat. Sometimes poor routing of a steering hose can cause the line to chaft against something else and cause a leak. Either way the only to repair the condition is to replace the leaking part.and topping off the with the correct fluid can also be just as important.
It sounds as if its not power stearing fluid but differentail fluid. Get this seen to aspa as if it is diff fluid and you ignore the problem, the back wheels will eventually seez up (like you've locked on the hand brake) and it will cause you to spin out and possibly crash.