The Yalu River border was the only thing that the Chinese had a right to bitch about. One we never crossed. Unfortunatley. As for it being a surprise, obviously it was. It was regarded as bluster, pure and simple. All of their military movements were kept completely secret.
And it was very stupid. One of the results of the contempt for your opponent is that you lose the ability to predict his moves.
Half of the chess game is calculating enemy's intentions. You do it by imagining yourself in his position - a taboo for those who think that oponent is crazy, subhuman or irrational.
Chicoms just have emerged as victors from the long war against old regime and Japanese invaders. The northern part of Korea was the most critical point of possible new invasion or leverage for destabilizing foreign subversion.
Chicoms HAD to act, at first by sending warning signals and next by sudden "sneak" attack. EVERYBODY in their place would do exactly the same thing and failure to predict it was criminally negligent.
The outcome of Korean war - the division into two stable spheres of influence between USA and Red China could have been achieved without bloody war.