How can double jointing occur in babies?

Relaxin: Pregnant moms make a hormone called relaxin in their bodies. This helps their ligaments to soften to allow their pelvis to expand during child birth. This also allows baby's joints to be hyper-flexible as well. Combined with the cartilaginous (and highly flexible) skeleton of a newbie, you have the "double-jointing" to which you refer.

Hypermobility: Most babies have loose joints compared to adults. Hypermobility, which is a range of movement far outside the normal range can be a benign finding, can occur in Down Syndrome due to low tone, or can be due to connective tissue abnormalities such as Ehlers Danlos Syndrome or Marfan Syndrome. These are often inherited and the presence of a family hx will help in the diagnostic process.