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There could be multiple reasons for piped IO streams in Scala hanging without end. Some of the common reasons are:

1. Deadlock: If the output buffer of the piped input stream becomes full, and the reading thread is waiting for the input thread to read from the output buffer, it can cause a deadlock. Both threads are waiting for each other, and the program hangs.

2. Race condition: If the reading and writing threads are not synchronized properly, it can cause a race condition. It means that two threads are accessing a shared variable simultaneously, and the output is unpredictable, leading to the program hanging.

3. Closing streams: If the input or output streams are closed prematurely, it can cause the program to hang. It happens because one thread is still waiting for the other thread to write or read from the stream, but the stream is already closed.

4. Large data volume: If the piped streams are used to transfer large volumes of data, it can cause the program to hang due to high memory consumption or buffer overflow.

To avoid such issues, one should use appropriate synchronization mechanisms, handle exceptions gracefully, and optimize the performance of the program by using proper buffer sizes.