Ask your neighbors if they have a similar one in their back yard. If so it may be a flood water runoff pipe. If it is the city or county records will show what it is used for. If not they can check the permit records to see if any legal work had been done on the property and for what reason. The last option would be that the pipe was buried before the property was developed or after the house was built to get rid of the pipe the cheapest way by burying it.