There's a tendency to think of alluvial fans as an arid landscape landform, and they often are. But they can occur anywhere debris flows are a major component of sediment transport. (I almost called the phenomenon "erosion," which technically, debris flows aren't.) And volcanoes generally are highly vulnerable to debris flows; in that setting, they're called "lahars." Volcanoes are steep, catch a lot of precipitation as a result of orographic lift, and often have large amounts of unconsolidated to poorly consolidated rock. It's clearly a setting where saturated ground can fail, leading to a torrent of mud to boulder-sized sediment along with trees, houses, cars, and any furry creatures unfortunate enough to be in the way. I suspect most of what we're seeing in this mini-fan is the result of single rocks and boulders falling down the stream bed- not one or more lahars. As we'll see soon though, this whole area is composed of lahar deposits, young and old.
Followup, 3:33 PM- It just dawned on me, if this pile formed the way I described, it should more accurately be described as a talus cone.