The main obvious one is the reaching to grab the bottle while rolling. Your hand can and WILL get caught by the spokes causing an accident in more than one way injuring more than just your hand. Another is the clear possibility of dropping the bottle right in front of the rear wheel and crashing. Another is the real possibility of getting your arm caught with the handlebars and twisting/turning the bars by mistake while reaching down = crash again. Lastly, I see the fact that the weight of the bottle with liquids will add a huge instability factor to your front end making it dangerous when turning, taking a hand or two off the bars, etc. Just too many risks.

Many have already given excellent suggestions. Look into those first and get rid of this system before you hurt yourself.