Hello, jonah89,
The whole translation would be: "because R.R. has cut it to it (that is to say "has cut the tail to the dog". So, SE: to the dog, and LO: the tail.
LE (indirect object) becomes SE when followed by the direct object (LO/s, LA/s).
R.R. LE ha cortado el rabo---- R.R. SE LO ha cortado.