It’s because I’ve had many installs fail when they have been InstallShield-based installations.
What would happen is this:

1. Start setup.exe in silent mode.
2. Application would install.
3. Setup.exe finishes and exits.
4. I start the next setup.exe.
5. It fails “an InstallShield installation program is already running”.

What the????
At point 3, setup.exe HAS finished.  But it has left a cleanup program running, such as _isdel.exe.  The next setup.exe (at point 4), sees this and stops with an error message.

The workaround?
Write some code like this, to wait until isdel.exe exits.
while process "_isdel.exe” exists
sleep 5 seconds
loop