The Torah tells us that the when the Kna’anim heard that Aharon died and the Ananei HaKavod went away, they attacked Bnei Yisroel.
Rashi tells us that this was really Amalek who spoke the language of Kna’an although they were dressed like Amalek. Why would Amalek attack again after having been truly beaten the first time? Why did the disappearance of the Ananei HaKavod give them hope? Why did they speak Kna’ani?
Rav Yehonoson Eibushitz answers that Bnei Yisroel did not perform the Mitzva of Bris Mila in the Midbar because the Ananei HaKavod prevented the Ruach Tzefonis, the northern wind from blowing. This wind is necessary for the healing process and without it having a Bris Mila would be too dangerous.
Now that the Ananei HaKavod went away, Amalek was sure Bnei Yisroel performed Bris Mila at the first possible opportunity. They figured that in Bnei Yisroel’s weakened state after the Bris Mila, they can defeat them just like Shimon and Levi did to Shchem.
The reason for the attack was that Amalek was a grandson of Eisav and they wanted to avenge Yaakov’s stealing of the Brachos. To receive the Brachos from Yitzchok, Yaakov wore Eisav’s clothing but spoke in Yaakov’s voice. Therefore to raise the level of passion for the fight they chose to carry the banner that would remind them of this episode and they as well wore one nation’s clothing and spoke another nation’s language.