Splitwave function

I want to split a 2D wave into many 1D waves and so , I used splitwave function. I used the flag to overwrite the destination waves, but every time I run the code, it generates files with different names. I couldn't understand the reason. Could some one let me know the bug in the command? Here is the command:

splitwave/o yearly_data

yearly_data has 10 columns and close to 9000 rows.