Out of hatred for the Jews, of course. The claim is dubious, but revealing in many ways. From the Ansar al-Mujahideen site (thanks to Dan):
Allah Almighty says: (And wage war on all of the idolaters as they are waging war on all of you) 36 Surah At-Taubah
Allah have granted a group of our brothers from the lions of Masadat Al-Mujahidin at 8:00 in the day before yesterday Thursday 25 Safar 1433 A.H. corresponding 19 January 2012 to set fire in the forests of Nevada in the Reno region.
And we by this great event and paradigm shift declare the following:
1. we declare our full responsibility of setting fire in the Nevada forests in America.
2. we declare widening the area of war and transferring it currently to inside America and soon to elsewhere.
3. we give the enemies of Islam and the allies of the Jews who occupy the land of Palestine three months beginning from the date of this statement to disown from the Jews who occupy the land of Palestine, and their actions against our Moslem brothers, and we demand the end of their alliances that oppress our rights as owners of the land, or we will be forced to extend our war until it spreads in all the lands that plot with our enemies.
4. we emphasize that targeting the so called Israel and its allies headed by America in every kind of targeting including fighting and all the types of peoples civilian and military is an individual obligation (Fradu Ayin) on every Moslem until the liberation of Palestine, and all their armies are expelled from the Moslem lands, and Allah’s Word become the uppermost and religion is all for Allah.
5. we also point out that fighting the Jews and their allies is easy to those who want to repel their evil on the Moslems, and here you see with your own eyes what simple materials can do, that are cheap in your enemy, and how much damages it can inflict in them, so we incite the Moslem Ummah to participate in this blessed work and move their inherent energies for that.
This blessed Gazwa comes within a series of Gazawat that we declared and have previously begun and which will Insha”Allah destroy the edifices of the hateful Jewish occupation, until the Jews return from where they came since they have no place among us, and until the global alliances with the occupying Jews who occupy the land of the Moslems.
May Allah bless the soldiers of Al-Rahman and lions of Tawhid, may Allah aim their shooting and steadfast their feet, and accept from them their jihad and good deeds, because they made the friend happy and annoyed the foe.
And we say to the Jews and their agents this is a disgrace for you in this Dunya, and you have a big punishment in the hereafter, so have tidings with what you hate we have for you more, and do what you want, your cunning won’t benefit you and your scheme won’t benefit you, either we live under the governance of Allah or we seek shade under the Shadow of Allah.
ÙˆØ§Ù„Ù„Ù‡ Ø£ÙƒØ¨Ø± Ø§Ù„Ù„Ù‡ Ø£ÙƒØ¨Ø± .. ÙˆÙ„Ù„Ù‡ Ø§Ù„Ø¹Ø²Ø© ÙˆÙ„Ø±Ø³ÙˆÙ„Ù‡ ÙˆÙ„Ù„Ù…Ø¬Ø§Ù‡Ø¯ÙŠÙ†.
ÙˆÙŽØ§Ù„Ù„Ù‘Ù‡Ù ØºÙŽØ§Ù„ÙØ¨ÙŒ Ø¹ÙŽÙ„ÙŽÙ‰ Ø£ÙŽÙ…Ù’Ø±ÙÙ‡Ù ÙˆÙŽÙ„ÙŽÙƒÙÙ†Ù‘ÙŽ Ø£ÙŽÙƒÙ’Ø«ÙŽØ±ÙŽ Ø§Ù„Ù†Ù‘ÙŽØ§Ø³Ù Ù„Ø§ÙŽ ÙŠÙŽØ¹Ù’Ù„ÙŽÙ…ÙÙˆÙ†ÙŽ.
General command of Masadat Al-Mujahidin
Saturday 27 Safar 1433 A.h.
Corresponding 21 January 2012