A barrel or cask of wine is spilled on accident. Wine can symbolize blood because of it's similar hue. Wine has much significance to the French. In this case, wine would symbolize the blood and manslaughter caused by the French Revolution.
Wine spills onto the streets, attracting and gathering the common folk. They try to gather as much as they can. The way they scramble to collect even the smallest drops symbolize hunger. The people are starved and desperate, symbolizing the revolution. A man even writes blood in this wine!
It is assumed that the wine was meant to be delivered by the merchant to a shop. The owners are Madame and Monsieur Defarge. They are known for being heavily involved with the French revolution with their house and shop a large source of activity.