Here are two possible ways Ebola could become a pandemic:
1. It could slow down.
In the past, Ebola patients were only infectious when they were very ill and obviously infected with this disease. If the virus were to change in such a way that patients were well enough to travel while they were infectious, they could spread the virus to others before they died, especially if it was not obvious that they had Ebola. There are anecdotal reports that this may be happening in the current outbreak in West Africa.
2. It could become airborne.
In the past, Ebola was spread strictly by bodily fluids. If it changed so that it was transmitted through respiratory droplets, Ebola might spread to others much easier. There is no evidence of this in the current outbreak. However, another strain of Ebola, the Reston virus, was shown to be capable of respiratory transmission. So conversion of any Ebola virus to a respiratory form is a theoretical possibility.