I have been noticing a lot of misconceptions on the ground related to the functioning of the black gates. There have been quite a few theories being floated around creating a lot of confusion. No, jumping on one leg in front of the gate while holding your left ear will not make it spawn!
Here is the truth of the matter,
1) The spawn at gates does not depend on how many people you have present to start up and for all purposes, it is random. The mercenaries may attack without any warning. Scouts have been seen roaming the lands in the middle of the night!
Conversely, often you may find the gates completely quiet as well. However, the invasion will last for several days and hopefully, everyone will get a shot at it sooner or later. Do keep in mind that we are in initial stages of the invasion and as such, you will not run into a major attack that often.
2) There will also be times when you have advance knowledge of an invasion due to information collected from the books that a mercenary may drop. Keep an eye out for such information by communicating with others in game or through the various shard forums. There is a nice thread on Baja Stratics which attempts to collect this very information.
(Fiction based explanation)
The mercenaries are unable to do a full-fledged attack due to the limitations of the black gate system. So, they are relying on surprise attacks at seemingly random times to overwhelm their targets while they work on bypassing the limitations.
Also, once again, if you have a comment or suggestion, please use the feedback link as it is the fastest way to get to us.