AFAIK the ICTS stickers are, as GUWonder states, simply used to indicate that the passport holder has answered the random and pointless questions that the US Government requires be asked.

The trigger for the extra security usually comes in the form of an SSSS notation on the boarding pass. If your parents still have their boarding passes, take a look to see if they have that notation on them. I have no idea what triggers an SSSS notation (it's kept confidential) but this would be done by officialdom. Otherwise, the extra security should be truly random.

If I think back to the few times I've had extra official attention of that nature, on each occasion it has been when there was some reason why I had to interact with one of the officials by asking an extra question, or when I was late to the gate. My guess is that's some form of trigger - perhaps they think that the question or lateness has been thrown in by the passenger in order to distract attention. If that's the case, then asking for a wheelchair from the agents almost certainly was the trigger.

And, yes, a different coloured sticker could well be the flag that the agent wasn't comfortable with his interrogation.