Muhammad violated the sacred months by killing and looting caravans.
Ibn Kathir:
The Quraysh said that Muhammad and his Companions violated the sanctity of the Sacred Month and shed blood, confiscated property and took prisoners during it. Those who refuted them among the Muslims who remained in Makkah replied that the Muslims had done that during the month of Sha`ban (which is not a sacred month).
Ibn Ishaq/Hisham 424, Ibn Kathir V.2 p.242:
[A Muslim raider] who had shaved his head, looked down on them [the Meccan caravan], and when they saw him they felt safe and said, "They are pilgrims, you have nothing to fear from them."
Ibn Ishaq/Hisham 425; see also Ibn Kathir V.2 p.243
The Muslim raiders] encouraged each other, and decided to kill as many as they could of them and take what they had. Waqid shot Amr bin al-Hadrami with an arrow and killed him...