Poole was Killed by HAL.
As the entire ship and mission is under complete control of HAL then it can basically do anything.
My read on the situation is that HAL caused the pod to 'attack' Poole, thus disconnecting his oxygen tube and 'shoving' him off into space. Because in the 2001 universe, the rules of physics are strictly adhered to (unlike most sci-fi), once Poole was on his way there was no way to stop him, let alone catch him.
In the book, there's an line after Poole is killed saying (something like) 'Frank Poole became the first person to reach Jupiter'.
I wasn't quite right !
Dave Bowman did catch Poole but not before his oxygen ran out. See this from Wikipedia:
In an act resembling human desperation, HAL rams Poole with one of the spaceship's EVA pods, severing his oxygen hose and sending Poole hurtling into outer space without his oxygen. Bowman, in a second pod, races from the Discovery One to retrieve Poole, but is unable to reach him before Poole runs out of oxygen.