Crucifixion in Roman times was applied mostly to slaves, disgraced soldiers, Christians and foreigners–only very rarely to Roman citizens. Death, usually after 6 hours–4 days, was due to multifactorial pathology: after-effects of compulsory scourging and maiming, haemorrhage and dehydration causing hypovolaemic shock and pain, but the most important factor was progressive asphyxia caused by impairment of respiratory movement. Resultant anoxaemia exaggerated hypovolaemic shock. Death was probably commonly precipitated by cardiac arrest, caused by vasovagal reflexes, initiated inter alia by severe anoxaemia, severe pain, body blows and breaking of the large bones. The attending Roman guards could only leave the site after the victim had died, and were known to precipitate death by means of deliberate fracturing of the tibia and/or fibula, spear stab wounds into the heart, sharp blows to the front of the chest, or a smoking fire built at the foot of the cross to asphyxiate the victim.
From - The history and pathology of crucifixion
And when the body was pierced to ensure death, it was seen that blood and water flowed from the wound, showing that the body was dead prior to piercing, the blood having separated into blood and blood plasma.