Probably a big salmon run in a place like that.  The sow and cub were probably just one of several bears around that creek.  Leading tourists along that creek at this time of year would be asking for a bear encounter.  

It's very possible that the sow was reluctant to move her cub because she was being herded toward other boars that might kill it.  As far as shooting the bear, difficult to say - they might have stumbled on it with little warning.  At close range with a bunch of tourists around and inexperienced guides, pepper spray is likely the better option.

Regardless, glad everyone survived.  Hope the guides come out OK.