What's causing initctl stop to fail?

I have an upstart process which terminates all child processes during post-stop:

-stop exec killall my-process & exec sleep 10

This works as expected, however I'm unsure of the following two cases: If the killall process isn't run as a background process:

-stop exec killall my-process && exec sleep 10

Or if I remove the sleep process altogether:

-stop exec killall my-process

In these two scenarios if i run initctl stop, it returns the following error:

: Job failed while stopping

Why does the first example stop as expected, whilst the other two examples fail to stop properly?