You are probably getting water / snow / ice, into your ground fault outlet or underground wireing. This is a huge problem with GFCI's. Cover your outlet with a water proof box / bucket during the winter and this problem should disappear. The heater itself will not trip the outlet unless the circuit is under sized, it fails internally, or the heating element is exposed to air while plugged in and causes it to short out. I have also seen underground wiring having a small nick in a wire, and only trips the GFCI when the ground is saturated with water. I do not use GFCI on my pond circuit due to this issue. If you do not use GFCI, you must be VERY careful to unplug all units before working on your pond. Check your local codes to see if there is any way to remove the GFCI from that circuit.
Is your underground wiring in a conduit or buried romex? If it is romex, that may be the issue, and may need to be replaced. Your underground circuit may have been compromised by digging, landscaping, etc, the year you started having problem.