After his death, a team of delvers sacked the lair and with heavy looses, finally defeated the guardians and then proceeded to loot the place clean.
That would have been the end of it, but during the looting, the party's mage found the sorcerer's spellbook. It contained instructions on how to create more of the insidious fiends.
The mage then made copies and sold the spell to other sorcerers and bopclowns spread, until they became the scourge to delvers that they are today.
Bops are usually deployed to guard or block access to specific areas, and can remain deflated and dormant for many centuries, long after their original purpose has long since eroded away. For that reason they can be found almost anywhere. When disturbed, they inflate with incredible speed, ready to block trespassers and would-be robbers.
Most are created with a password or are tuned to a biothaumatical signature that allows a person to step past the creatures without incident. Most bops do not attack directly, but anyone not cleared are simply blocked from moving forward. It is only when attacked that the construct displays its true powers.
Anytime the creature is attacked, it immediately responds with equal force back upon its attacker. The harder you hit it, the harder it responds. In addition, most are impervious to blunt weapons and many simply deflate if attacked from a distance.