Unfortunately this happens quite a lot. Have known several people on visas and even a couple on green cards whose adult kids have been denied entry. They suspect immigrant intent, intent to overstay and/or intent to work for the parents business, rightly or wrongly. One couple I know who were green card holders have given up their green cards and gone home because of just such a problem, basically their son was not able to visit them again. It is hard to persuade the embassy otherwise once they have got the idea in their heads. One of the problems is you are more likely to get a B visa if you can prove really firm ties to the UK such as owning property but of course few young people can afford to do this nowadays.