A recent scientific paper by the The Geological Society Of America titled: Future volcanism at Yellowstone caldera, suggests that previous eruptions have come from deep within the volcano and they have occurred very rapidly.
The paper says:
These source regions coincide with the upper parts of the present-day imaged magma chamber, while the faults focus much of the present-day caldera unrest.
Based on these combined observations, we propose that volcanism has a higher probability to resume in three fault-controlled NNW-trending lineaments, the first coinciding with the western caldera rim, the second lying across the central region of the caldera, and the third extending across the northeastern caldera.
The first two lineaments focused recent intracaldera volcanism (174–70 ka), while the latter is the most active in terms of current caldera unrest.
Future volcanism could include large-volume lava flows and phreato-magmatic rhyolitic eruptions. The identification of these three regions together with potentially rapid eruptive mechanisms may help to better define future monitoring efforts necessary to improve eruption forecasting in this vast area of volcanic unrest.
What this means is that science is only beginning to understand just how volatile Yellowstone is, but doesn’t yet know how it will next erupt or when.