Why visiting wife and kids are interogated at the airport when they have visited before

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My wife and kids ( 3 and 6 years old ) live in Botswana. In 2016 November they visited me until May of 2017 and returned to Botswana. They returned on time to avoid overstaying. I have wanted them to come again in December of that year for another period of six months but they where not allowed to board the plane in South Africa causing stress to the entire family. I contacted my local councilor and MP who helped me a great deal. I finally got compassion and the tickets from my money back. I bought my wife and kids some tickets to visit me on the 19th of December so I could spend christmas with them. The boarder agency did not let them in and are still holding them and questioning them and yet I have provided them with all the documentation listed in the .gov website that they require to come into the UK. I m stressed and I just don't know what to do. Feeling very suicidal. I just don't know how my wife and kids are feeling but they must be stressed. They landed at 05:25 and I m still waiting for communication from the Border Agency. Surely this is not fair for them to put my family and  me through such inhuman treatment.