The reason for God's anger against Israel is unclear (but see Deuteronomy 4:25; 6:14–15; 29:22–28; 31:16–18).
The phrase stirred up refers to a Hebrew verb (suth) that means enticing or inciting someone to do wrong (1 Samuel 26:19, stirred you up against me). People usually disliked census-taking because they saw it as an invasion of privacy. They thought it led to taxes or forced military service (see study note on 2 Samuel 24:9). If David acted out of pride or was planning an unjust war (as 2 Samuel 24:9 might suggest), this could explain why it was considered a sin for him (2 Samuel 24:10).