He also happens to be doing his strength training up on a secluded cave in a mountain under a waterfall (think fighting anime training tropes.) He looses his balance while lifting and plummets to the rocky pool below. Bob is at summer space camp.
Bob doesn't realise the camp is actually in space. Didn't bring a spacesuit. Bob is attempting to fix his toaster.
Bob is hit in the eye by an errant mortarboard, and dies of an infection a week later. Bob is attending a concert.
Bob is the only one not to move when one of the performers decides to stage dive. Bob is digging up some carrots.
Bob gets told something no-one was meant to know, gets ejected from the Universe. Bob burns the land and boils the sea.
Bob's brain slowly turns into mush from all the un-funny twelve yearolds yelling, "WHAT ARE THOOOOOSE!?" Bob finally decides to end his suffering by commiting suicide.
A passing goddess transforms Bob in to Barbra and forces if to forget his old life. (So for all intent and purpose Bob is dead and someone new lives in his body.) Bobbie joins SHIELD.