Doctors are still very confused about the root cause behind bleeding in Ebola patients. This is not a very common occurrence, said to be roughly prevalent in only 18% of ebola victims, however, these cases are usually very severe.
If bleeding does occur, it is generally in the later stages of the disease.
Ebola spreads through the body the same way most fatal viruses do, starting by affecting the immune system, eventually breaking it down, then growing exponentially until it interferes with the regular function of the organs.
It is thought that the ebola virus might effect the proteins created by the liver that are used for blood clotting. Eventually, these are created in abundance and thus excess blood is the result. This could be related to why ebola patients sometimes suffer from bleeding. Again, the answer is still not exactly clear.
I hope this helps.