Unlike coast redwood, mountain hemlock are not known to grow back from sprouts after cutting or fire. But they can grow multiple stems when small seedlings suffer top damage - when they get browsed or damaged by snow and ice.

Also, hemlock's relatively light, wind disseminated seeds can cluster together like blowing sand or snow drifts.  Soil surface roughness or woody debris might cause seeds to collect in wind eddies or sheltered areas, leading to multiple seedlings coming up in a cluster.

Other species with larger seeds like pine and fir can get planted in clusters by squirrels who don't find all of their stashes.

Thanks for the response.  Multiple trunks growing from a cluster of wind- or water-dispersed seeds makes sense.  

Branches sprouting from lower portions of tree trunks are usually "epicormic" branches. They originate from dormant buds beneath the bark. These buds are typically suppressed by hormones produced by the active growing shoots higher up in the tree. Epicormic buds may be stimulated to sprout by exposure to sunlight as the forest is opened, or by damage to the top.