Chosen answer:When the two Orcs were fighting in the tower where Frodo was being kept, the larger one shoved the smaller one down the stairwell to a lower level. He then yelled down that the smaller Orc tried to kill him, so they should kill the smaller Orc. From that brawl an Orc was pushed out a window down to yet, another lower level. This spurred another brawl below...a chain reaction, in other words. Also, there were two companies of orcs in the tower so there was a natural rivalry already present - much like when the orcs and uruk-hai that captured Merry and Pippin fought amongst themselves. Finally, the power of the ring had an effect.
Chosen answer: When the two Orcs were fighting in the tower where Frodo was being kept, the larger one shoved the smaller one down the stairwell to a lower level. He then yelled down that the smaller Orc tried to kill him, so they should kill the smaller Orc. From that brawl an Orc was pushed out a window down to yet, another lower level. This spurred another brawl below...a chain reaction, in other words. Also, there were two companies of orcs in the tower so there was a natural rivalry already present - much like when the orcs and uruk-hai that captured Merry and Pippin fought amongst themselves. Finally, the power of the ring had an effect.