I think this behaviour is supposed to be a feature of shell. Pipelines are executed in parallel.

Also, see `sponge` from `moreutils` (http://kitenet.net/~joey/code/moreutils/):

    $ man sponge
    ...
    sponge  reads standard input and writes it out to the specified file. 
    Un‐like a shell redirect, sponge soaks up all its input before opening the output file. 
    This allows constructing pipelines that read from and write to the same file.