There has been a big change in the way landfills are used in the US.  More stuff is filtered out before it gets there (a higher portion of yard wastes are now composted), and care is made to keep moisture content (and therefore decomposition) down ... I don't know if this is the key, but it might be over the same timeframe.

I suggest one possible explanantion:

The thinning ozone layer has increased the UV radiation reaching the lower layers of atmosphere. The UV rays cause the formation of more OH radicals in the atmosphere, which react with CH4 and oxidize it (via a chain of recations) to CO2 and H2O:

Photolysis of O3 to O(1D) in the troposphere is determined by a narrow band of radiation in the 290-330 nm range, reflecting the combined wavelength dependences of the actinic flux, O3 absorption cross-section, and O(1D) quantum yield ( Figure 1 ). Radiation in this wavelength range is strongly absorbed by overhead O3 and hence the production of O(1D) is strongly dependent on the thickness of the stratospheric O3 layer [Madronich and Granier, 1992].

http://www-as.harvard.edu/chemistry/trop/publications/jacob2000/text.html#77032

The atomic oxygen O(1D) is the cause of formation of OH radicals in the atmosphere via the reaction:

H2O + O(1D) -> 2OH

Thus the more UV light reaches troposphere, the shorter the CH4 lifetime.

More seriously, here's a slide presentation on atmos. methane:

http://www.cmdl.noaa.gov/gallery/annual_meeting_2004_dlugokencky/Slide1?full=1