Swept back empennage
 

What is the reason that high speed aircraft generally have swept back horizontal & vertical stabilizers?

Probably the same reason they have swept back wings, to delay the onset of supersonic flow.

In some a/c, tail surfaces are given sweep to increase its moment arm. That allows having smaller and thus lighter tail surfaces while having nice stability and control authority. From the top of my head, you can see that in Yak-40 and Il-28, there are probably hundreds more.
Regards,
Andrés