What could cause coolant to leak while the ac is running? on 2001 BMW 330i

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It's been over 100 degrees in Denver/Boulder for the last week. We have been turning on the AC for the car but a few days ago, the light for the coolant level came on while driving and we got to the gas station and it was leaking. We refilled the coolant didn't leak after and today it was hot in Denver again over a 100 and we turned on the AC again and when we stopped the car and did our shopping came back there was coolant on the ground. We haven't used the AC since. We want to know what could cause the coolant to leak.
(2) Answers
(1) Comments
If your using the a/c in stop and go traffic and your cooling fans for the radiator and condensor are not coming on your engine will over heat but because of inop. fans not because of the a/c.
Drain,flush and refill the cooling system with new antifreeze. Then replace the radiator cap As antifreeze ages it lowers its boiling point which will cause steam and increase pressure in the cooling system which in turn will push coolant out the the overflow. If t here is enough pressure more then 16lbs it will blow out the overflow bottle.
that sounds way to technical.