Charles Ellicott Commentary Deuteronomy 32:27

"Were it not that I feared the provocation of the enemy, Lest their adversaries should judge amiss, Lest they should say, Our hand is exalted, And Jehovah hath not done all this." — Deuteronomy 32:27 (ASV)

Behave themselves strangely. —Possibly, misunderstand it, or take note of it (as a strange thing).