The Lake, which is the location of last earthquake swarm and an underwater bulge atop the caldera, has been doing this for a few days now -- intermittent, abrupt shakes. Notice that the seismometer's sensitivity is set at 500 microvolts, as opposed to 50~125 at most other stations in the park. So the shakes would max out at 100 microvolt sensivity.
These shakes are local to the station, and obviously not seismic. My (amateur) guess is there've been quasi-periodic magma movements or water sprouts at the lake bottom. Notice that Old Faithful sprouts don't register anything at the YFT station set at 125 microvolts. So these bursts are likely at least a hundred times stronger than Old Faithful.