That is because they said to those who disliked what Allah sent down: "We will obey you in some of the matters." Meaning: in associating partners with Allah, they agreed with them upon shirk (polytheism) in secret, "and Allah knows their secrets."
Muhammad said: Whoever recites it with the fatha (vowel sign) on the alif, it is the plural of 'sirr' (secret).
﴿ذَلِكَ بِأَنَّهُمْ قَالُوا لِلَّذِينَ كَرِهُوا مَا نَزَّلَ اللَّهُ سَنُطِيعُكُمْ فِي بعض الْأَمر﴾ أَي: فِي الشّرك وافقوهم على الشّرك؛ فِي السِّرِّ ﴿وَاللَّهُ يَعْلَمُ إسرارهم﴾.
قَالَ مُحَمَّد: من قَرَأَ بِفَتْح الْألف فَهُوَ جمع (سر).
