Why was Yehoshua incensed?
Rashi #2, Ramban #2 and Targum Yonasan: Because they were prophesying that Moshe would die and that he, Yehoshua, would take Yisrael into Eretz Yisrael. 3
Ramban #3: He thought that they were prophesying from the Atzilu of Moshe, together with the seventy elders, in which case they ought to have prophesied beside the Ohel Mo'ed, like they did, in which case they had no business to prophesy independently in the camp. 4
Seforno: Because he suspected them of deliberately withdrawing from the lottery in order to show that they could prophesy independently, without Atzilus from Moshe.
Ramban: Consequently, their prophecy must have been false.or because they were guilty of Paskening in the presence of their Rebbe.
See Ba'al ha'Turim, on Pasuk 27.
See Ramban, DH 'u've'Targum Yerushalmi'.
What did Yehoshu'a mean when he said "Kela'em!"?
Rashi #2, Ramban #1, Rashbam and Targum Onkelos: He meant that he should (stop them 3 and prevent them from going out into the street
See Sifsei Chachamim.
Because he thought that, since they had withdrawn and not been appointed elders, they had no right to prophesy.
Rashi: Because they were prophesying that Moshe would die and that he, Yehoshua, would take Yisrael into Eretz Yisrael. Ramban: Because they were prophesying falsely and needed to be jailed, like a crazy who walks around prophesying.