If there are from opposites sexes, there is certainly an egoistical reason for the rat to free the other.
If they are not, rats are very sociable. They need close contact with other rats.
It also looks like the opener rat is also very curious and wants to enter the cage too to know how it feels inside.
There is a huge tendency to build up story around a single video, but this is a repeated experiment with far more evidence than shown.
My issue is not with the results of the study, but with interpretation. I think it's possible to explain the desire to free the other rat purely in selfish terms without any nods to altruistic or empathic behavior.
Reproduction is for gene survival. We are simply throw away machines our genes have developed in order to propagate themselves.
The source code (genes) propagation per se would be completely useless to the system, without the processes they express: living systems, the most efficient energy transforming machines the system has been able to incrementally arrange up so far starting out of random stuff.
Therefore reproduction is exactly for that, for allowing the continuation of this energy transforming processes (life processes) in the most efficient way.