Some of the key things to look for when a gate opener is opening by itself is the accessories / receiver. The number one cause is an exit sensor. They are very sensitive devices and will cause the gate to go open as if a car is triggering it when there is no car. Try disconnecting the device and see if the problem persists. If you don't have an exit sensor it could be the receiver. Receivers could be picking up another radio signal in the area. Newer receiver coding and frequencies are making this less likely however it should still be checked in the same way by disconnecting and seeing if the problem persists. Lastly any other accessories (keypads, push buttons, etc) should be disconnected to see if the problem follows those devices. Many times if the devices start to build up corrosion internally they can malfunction and open the gate.