Strobes on TOO long

Going from 6 to 16 strobes is the problem. Each strobe has a large capacitor inside. It stores up a high voltage and the strobe will fire until that voltage drops too low to trigger the internal electronics. This is good and bad. You may have noticed that when the strobes have been OFF for a long time, they don't start strobing instantly and with multiple strobes, they will all fire at the same time for the first time. That is because the capacitors were discharged. When your controller cuts power to the strobe(s), they still have stored power. The more strobes, the more power. They will flash until that power is used. After your show has begun, they retain some charge and will immediately be "random" rather than give one big initial flash. 

To fix your problem, you can use fewer strobes per channel. Not a great option if you are channel poor and strobe rich.

Or, you will need to put something else on that channel that consumes their power, something like a C9 bulb or two. This still isn't perfect and you may get a few stray strobes, but it will reduce it a lot. Personally, I use white or clear bulbs, and it seems that the best location is in a socket between the controller and the first strobe. Hide it if you can but it may overheat the bulb and socket if you paint it black. Depends on how long you keep your strobes ON.

Others have plugged in spare wall-warts at the end of the C9 string and said that worked for them. They have transformers inside and transformers are the "opposite" of capacitors, capacitance vs. inductance, and would seem to cancel each other. 

ps, Always double check your sequences to make sure no shimmers, twinkles or low intensity effects are on your Strobe channel. They will damage your strobes faster than a neighbor kid.