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-TaubahAllah 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.