It was common Muslim tactics to get near UN troops and provoke the Serbs in order to draw fire towards the UN and get the Serbs in trouble. If you don't believe me, I can link to Hague testimony by a Ukrainian soldier who was in Sarajevo - testifying that he heard outgoing mortar fire next to the UN compound in Sarajevo which was located in the part of Sarajevo controlled by the Muslim army.
Anyhow, one earlier attack might not have been caused by the Serbs:
"In one incident, on March 18, Serb fighters destroyed a Swedish armored personnel carrier with an antitank missile and six tank rounds in the northern town of Gradacac. Moller reported it, clearly blaming the Serbs -- but U.N. officials in Zagreb contended that the source of the attack was unknown."
The Muslims did frequently try to coax the Serbs into shooting at UN troops. Gen. MacKenzie reported as much in Sarajevo. But the Serbs shot at UN troops a lot more than the Muslims tried to draw Serb fire.
It's kinda like the marketplace massacre in Sarajevo that many blame on the Muslims. It may very well be true - the Muslims very likely fired that mortar shell. But how does one dismiss the other few thousand shells that landed in Sarajevo? All fired by Muslims?
Likewise, every Serb shot at a UN post was not the result of Muslim fire.