Most babies start to roll "almost by accident", a typical example would be when they - placed on their tummy - lift their head too far up and sideways and "topple over", or, laid on their back, lift head and feet and topple sideways.
Even if they manage rolling in one direction, it takes a few days to weeks until they can turn back at will. Leading to the following scenario: Baby rolls over, can't get back, protests against position, parent bends down and turns baby, repeat after 30 seconds.
A swaddled baby will be even more hampered to change its position - imagine yourself landing face-down with your arms wrapped in a blanket. This can increase the risk of suffocation or, at least, be very frustrating. Depending on the type or method of swaddle, the child can even get tangled in the loosening fabric or wrap part of it around the neck or head.
If all these are not enough to stop swaddling a mobile baby, remember that one of the main intentions of swaddling is to calm very small infants by mimicking the restricted room they had before they were born. Babies that start to roll or move their arms and legs need the freedom to do so. (Note: IMHO and experience, this does not apply to carrying a baby in a sling or similar, especially as for older babies the arms stay unwrapped.)