Why are thrust faults older than normal faults in the Himalayan-Tibetan plateau?
In line with what strawberry-sunshine has answered, and to put it into more layman terms, after a compressive episode there's a distensive one.
Imagine you have a volume of sand, and you are compressing it horizontally in one direction, it is confined in the other horizontal direction and the only force acting vertically is gravity.
During compression, the sand will reorder itself in response to the applied stress: this reorganization are the equivalent of the thrust faults. You keep applying force until movement is negligible. Then you stop applying it althogether. The weight of the sand will force a second reorganization to adapt to the new conditions. This is the equivalent of a distensive period, and the displacements inside the sand mass are the equivalent of the normal faults developed in the second stage of the orogen.
Although reality is more complicated that that, I hope this example helps you understanding why normal faults can be younger than thrust faults.
Best regards