Power plane de-coupling

I am using SIwave to compute the eigenmode for a PCB board with 4 layer board (s/g/p/s). The results show that some modes' Q are only 2, and some modes do not show a significant strong local E field.

My questions are:
1. What cause the Siwave capture those low Q mode?
2. For some mode, why I can not see any strong local E field? Does it come from the ground/power plane circumference?
3. Why to decoupling them?

Thanks,
Newbenn