Why does it say "in Yosam's 20th year"? He reigned only 16 years. It should say the fourth year of Achaz!
Rashi citing Seder Olam 22 #1: Because Achaz was a Rasha, the verse prefers to count the years based on Yosam in the grave, and not according to Achaz.
Radak citing Seder Olam 22 #2: The decree 1 was made in the days of Yosam.
Malbim: Because Pekach fought with Achaz and wanted to end his kingship - "Na'aleh vi'Yhudah... v'Namlich... Ben Tav'al" (Yeshayah 7:6), his kingship is not counted from Achaz' years, rather, from Yosam's.
Also Yalkut Shimoni 236 says so. It is not explicit which decree they discuss. Perhaps it was that Pekach ben Remalyahu will kill 120,000 from Yehudah. (PF)
