It may not be anything to do with how long you're pushing the button, it could be that you've got cheap springs that shouldn't be used the way you're using them. OK, here's the deal:
Everything has some amount of resistance to electricity flowing through it. Some things (most metals) have a fairly low resistance - electricity can travel through it easily. But whenever you have resistance, some amount of the electricity flowing through turns into heat. If you force more electricity through metal than it can handle, the heat generated damages the metal. Heat damaged metal shows a color change, and it loses the temper of the metal.
A regular extension cord works great when you're using it to power a lamp. A lamp doesn't require much electricity. Take that same cord and use it to run your electric dryer, and you'll start a fire. You're pushing more electricity through the wire than it can handle, and that generates heat. Same thing with your spring. By using a higher voltage, higher current battery and combining it with a low resistance bridge, you're increasing the amount of power going through the spring to power the bridge. More than the spring can handle. Maybe that spring would be fine if you used a high resistance bridge. Maybe it would be fine if you used a lower power battery. But the combination you've got is too much for it. This assumes there are no shorts and everything is working the way it's supposed to.