As quoted from this wikipedia article, it states:
If a consul died during his term (not uncommon when consuls were in the forefront of battle) or was removed from office, another would be elected by the Comitia Centuriata to serve the remainder of the term as consul suffectus, or suffect consul.
In the book 'Augustan Rome 44 BC to AD 14: The Restoration of the Republic and the Establishment of the Empire (The Edinburgh History of Ancient Rome)' by J. S. Richardson - he quotes Suetonius as saying:
The reason he held the consulship now was, so Suetonius tells us, for the introduction into public life of Gaius Caesar, and, given the celebrations that attended this, this is no doubt correct; but it is worth noting that the same year also saw the reintroduction of the election of suffect consuls taking office after the elected consuls stood down, a practice that had last been used in 12 BC, The year that Agrippa died.
He also says:
From now until the end of Augustus' reign the election of suffect consuls was to be the norm, with exceptions only in 3 BC and AD 14.
This would suggest that quite often during the Principate consuls either died, stood down or were removed from office.
Why is it that there were so many suffect consuls during the Principate?
You can find a list of Roman consuls here.