SoftBank has been accumulationing stock. They have been trying to make Sprint profitable. It seems that they will forgo a profit short term in order to build up the customer base.
After the next presidential election takeover and merger rules may ease up giving Sprint a chance to merge or be bought out by a major player. That does not have to be a telco nescessarily. These scenario's may explain SoftBank's further stock purchases and the push to increase the customer base which makes Sprint more desirable.