What are the natural causes for methane formation?
The 'natural causes' of global methane emissions are, as you indicate, mostly of biological origin. To some extent it depends upon what you regard as 'natural'. There are many and differing estimates of global methane generation, of which I list a summary below. Those in bold you may regard as abiological. Values are in Mega-tonnes per year released to the atmosphere:

Wetlands 217

Fossil Fuels 96

Ruminants 89

Landfill 75

Geological sources 54

Freshwater 40

Biofuels 38

Rice growing 36

Oceans - DMS  20

Termites 11

Wildfires 10

It's a moot point as to whether degradation of dimethyl sulphide (DMS) is biological or not.