I'm a little confused by this question because it seems to me that there is only one reason why Telemachus asks the Ithacans who are his mother's suitors to convene an assembly.
The reason that he calls this assembly is because he does not like the behavior of the suitors. He does not like them staying in his father's house and, essentially, eating up all his wealth. He invites them, instead, to go have feasts at each others' houses.
If I had to identify another reason why he calls them, I'd say it's because Athena persuades him to do it. But she is really only encouraging him to do what he already wants to do.