Rashi on II Samuel
Adonoy's anger was again kindled against Yisroel. I do not know what it was [they did].
Ralbag on II Samuel
...The sin was in [the fact] that David, the anointed one of the God of Jacob and the sweet singer of Israel, placed his trust in the flesh of his forearm - upon his [having] many people. And it was not fitting that he should place his faith 'in many people [that] is the glory of the king,' but rather only in God. For there is nothing that prevents Him from saving through the many or the few...