What's going on in this "confusing" TCAS Resolution Advisory video clip?
The TCAS responded to the other aircraft not following the flight profile that was observed earlier. If you watch closely, you will see that the other aircraft started to descend as well after the first resolution advisory is given. 
The onboard TCAS responds by issuing the counter-movement to climb immediately.

The only thing that we do not know is what version of TCAS this is supposed to simulate, because newer TCAS systems can interrogate each other and issue resolution advisories not only based on own observed data, but also on data received from the opposite TCAS.

Related Question: How does TCAS work?

The entire situation could have been part of the training scenario, as this was done in a simulator, according to the YouTube description:


  Practising TCAS RA manoeuvers during our MCC course in a Boeing 747-200 full-flight simulator. This one was quite confusing when the TCAS' voice advised to climb just after having asked to descend.