Electron (fermion) is an elementary particle just like positron and they interact via boson which is photon, electromagnetic force. I like to know why is gluon (hadron) being emitted when electron and positron annihilated each other?
In layman terms, because the annihilation of the electron and the positron can produce a virtual photon which does then materialise a pair quark-antiquark. Then each of them end up producing bunches of hadrons because naked quarks cannot exist. More technically, this is the following Feynman diagram (and others but this is the dominant one).