If you heard a snap, most likely that sound is from one of the struts gas pressure leaking out.
However, i dont know how rain could cause this.....mabey water got between the seals of the gas struts and made that pop/snap noise you heard and the struts are fine after it dried out?
Also if one of your struts failed....they are filled with compressed gas by the way
....then the lift gate would have trouble opening properly cos it would be a uneven lift...ie heavy to open and easy to close if you dont use the power option.
How about you take some soapy water and spray it on the strut...if you see air bubbles then there might be a leak.