What does it mean when the depth of earthquakes get near the surface over a period of time?
The data from USGS is somewhat misleading.

Determining earthquake depths is a difficult problem, particularly if you're using data recorded a long way from the earthquake. Event location methods often become unstable for very shallow earthquakes. With some methods/catalogues it is therefore normal to "fix" the depth of shallow earthquakes, and not attempt to find a more accurate depth. This is why you see so many earthquakes supposedly at 33km; I think the 10km ones may also be fixed. Really, this ought to be made more obvious in the USGS site (I think the information is there if you dig deep enough!).

So, it's impossible to draw any conclusions from these graphs: you're probably just seeing improvements in seismological techniques over time.