I may has misinterpreted what the issue is. If the plug connectors are coming off of the actual spark plugs, the first approach would be to squeeze the boot near where the metal circular clamp is and re attach. The plug should not rely on the rubber of the boot holding the connection tight. If tightening doesn't help, you need new boots and as I mentioned.
The boot is there to keep water out and in fact the motor would run perfectly well with no boot and just the connector pushed on but over time, moisture will start corrosion and if the connection and plug were to get too wet, arcing from the connector to ground could occur and in effect kill your spark.
It is amazing how often I hear of plug wires coming loose. I suspect that they were either not pushed on all the way or they have lost their gripping ability.
When removing plug wires, pull straight out and try not to wiggle the boot to get it off (twisting is OK ). That just opens up the internal clip.