When the serial port is opened, or closed, the Arduino resets.
When you echo something to the serial port that way, it may be opening the serial port again, causing a reset.
The y with the two dots over it is caused by printing a -1 as an ASCII character. The -1 comes from trying to read data from the serial port, when there is no data to read.
It does not appear that that should happen, given the code that you are running.
Does the "serial test 0021" get printed every time you echo something to the serial port?