By "not using", I'll assume you don't have any set up in the "Joystick Options" either.
There is no default hotkey to close VoiceAttack (aside from something like Alt-F4, which closes the current window, but that's a Windows keyboard shortcut).
Likewise, unless you have a "Perform a Window Function" action set up to close VoiceAttack's window, or a "Stop a Process by Name" to kill its process, there shouldn't be any actions that explicitly close VoiceAttack directly.
Of course, if you have a command that send a bunch of keypresses, they could theoretically do any number of things if you direct them towards VoiceAttack itself, but that is true for any application, and it'd be more likely that random things start opening, rather than the application closing entirely.
The last-executed command is not stored, so that information is not available to retrieve on the next startup, unless you were to specifically set up all your commands to write their name to file when they execute, for example.
You could have a look in the Windows event log to check whether the application is crashing: Right-click the start button, choose "Event Viewer" (or search for "Event Viewer" in the start menu, if you prefer), expand "Windows Logs" on the left-hand side of the event viewer, if it isn't already, and choose "Application".
Click "Find..." on the right-hand side, type in "VoiceAttack", and click "Find Next" to cycle through any VoiceAttack-related entries in the log. Crashes are likely labeled as "Application Error" "Error"-level events