This is a long standing minor bug in the game. The drivers level does not recalculate on experience gains, so what has happened is that you most likely reached maximum in all other stats first and experience last.
This does not have any impact on your drivers ability, so you have nothing to worry about. The only things the displayed level of the driver controls is what level of player can purchase them on the transfer market, and their training speed (if the level of driver is higher than your account, training speed slows down significantly.)
If you are absolutely set on having a level 30 driver for some reason, what you would need to do is leave one stat at 29 until experience is 30, then train that final stat.
