Tannerite explodes when hit by a fast moving bullet. Useful for marksmen trying to see whether or not they hit a target hundreds of yards away...and useful for people who like to play with explosives and make youtube videos.
The shooter probably intended to use the tannerite as a detonator for an ammonium nitrate (ANFO) bomb made from the ammonium nitrate he had in his possession. ANFO bombs are very powerful explosives but they are very stable and can only be detonated by explosive detonator
The shooter no doubt planned to use a ANFO bomb detonated using tannerite as a detonator ignited by a rifle shot
There may be one or more caches of ANFO/tannerite devices hidden in the area that the shooter never got to set off
Looks like he may have had an escape plan using explosions of fuel tanks and ANFO bombs to create diversions as well as mass casualties