Crusaders led by Peter the Hermit sack Semlin and Belgrade, forcing Byzantine troops to flee to Nish. The conflict actually originated with a previous group of Crusaders — several thousand under the command of Walter Sans-Avoir. The city of Belgrade decided to wait for instruction from Nish, which in turn asked for instructions from Constantinople about what to do with them all.
This meant not opening up a market for them, preventing them from buying supplies and food — all of which started running short well before an answer was heard from the capital. This meant Crusaders started taking what they needed, and sixteen were caught pillaging in Semlin.
When Peter the Hermit arrives with his 20,000 followers, they see the sixteen confiscated suits of armor nailed to the town walls. One thing led to another and soon the Crusaders were sacking Semlin, killing thousands of inhabitants.
Semlin is just across the banks of the Danube from Belgrade, and the Crusades are in no mood to let a little thing like a river stop them; soon Belgrade is abandoned because the residents there don't want to suffer their neighbors' fates and that city is also sacked.