What could cause the springs to break on a 2001 Audi A4 or make the shocks leak ?

The strut or shock as you put it uses oil compression to keep bumps on the spring and suspension. The answer to your question is the strut is worn out dirt gets on seal and destroys it oil leaks out spring takes a good pothole......... and here we are. Replace both struts and springs at same time as the other spring may be weak. Hope that helps. Also have your tech check all control arm bushings and ball joints at same time.

That's not true. You can't compress a fluid. There is a nitrogen bladder that keeps pressure on the oil, when a sock/strut is compressed, a piston with specifically sized orifices is forced through the oil providing resistance. Once the sock/strut loses nitrogen pressure or the oil seal, the piston can travel freely, not dampening any vibration from the road. Springs hold the weight of the vehicle, shocks/struts prevent the springs from oscillating when you're traveling at speed. Theoretically, if all roads were perfectly flat and didn't even have the smallest pebbles on them, you wouldn't need shocks/struts. Shocks/struts dampen the compression and rebound of the spring so the spring doesn't keep bouncing. So when a shock/strut gets worn out due to age, it can't dampen those oscillations and the spring gets a real workout and eventually fails due to fatigue.

The other spring won't be weak if that sock/strut hasn't failed.

I recomended twin spring replacement so they are the same age and strength,and you must not have fully read his question the strut is losing oil thus gas left long before that.