One of the challenges is to pack all those satellites in launch vehicle's payload adapter. The payload adapter on launch vehicle needs to be flexible enough to allow such varying number of payloads. In this mission, ISRO had to realize a new/modified payload adapter to carry those 104 satellites.

Next is to accurately inject all those satellites - either into their respective orbits if it's a multi-orbit mission, or with sufficient time gap and re-orientation between injections if it's a single-orbit mission.

This was a single orbit mission (all sats released into SSO). So, PSLV's 4th stage had to time those 104 satellite injections. It also need to re-orient itself for each injection depending on satellite's requirement. This mission had as many as 103 "separation events". All this logic is pre-programmed and executed by launch vehicle's onboard computer.

Check out QuadPacks lined up before the launch: http://blog.isilaunch.com/the-quadpack-line-up/

And, check out those QuadPacks mounted on PSLV payload adapter around primary payload: https://imgur.com/a/LM9Cp