It happens every year. Usually when it's hotter though.
According to
http://www.wired.co.uk/article/uk-heatwave-rails-buckle , it is a real problem, and it's simple.
Rails are made from metal and metal expands. When they are placing rails end to end, if they give them a lot of room to expand, they will not be safe for trains to travel along when it's colder and the rails contract. If they expand too much, they will buckle.
They can minimise the risk by stretching the rail before it is laid, but if they do this too much, the rail will not be safe for use in cold weather.
Apparently, on average, the temperature of the rail will be around 20 degrees C higher than the ambient temperature. The temperature of the rail will only increase as more trains pass over it, and apparently will increase even more if they speed up or slow down (friction presumably).
The problem is made worse if the track is not maintained well.
The speed restrictions are imposed to keep the track temperature within safe limits.
You might find that the only reason that we haven't had these restrictions happening for that long (maybe 5-10 years) is that Network Rail has not long installed the sensors, so we could have been happily travelling over dangerously hot track for years.