A hypothetical example should help to understand this.

Suppose you have been taking suboxone for several months, and you decide that you don't need it anymore.  You feel ok for the first day, though you will likely develop cravings for opioids.  Then one of your old buddies gives you a call, and he is still taking street drugs, probably a mixture of fentanyl and heroin.  He talks you into what you know is a bad idea, but all of us suffer from temptations of various kinds, and because the buprenorphine blood levels are trailing off in your system, you are actually more vulnerable to these temptations than usual. 

So, you use an amount that you estimate will be effective, but of course this is just a guess, and quite risky in itself.  But then you are disappointed to note that the effects are not as strong as you anticipated; the reason may be that the buprenorphine is still in your system and active in blocking the fentanyl.  You feel guilty and you don't want to be going down this path, so you don't take any more that evening.

Now it is eleven o'clock in the morning the next day, and you don't feel very well, and you still have some of that stuff your friend sold you leftover.  So now if you are not thinking too clearly, you may decide to go ahead, jut one more time, and snort a little, but maybe a little more than yesterday, because yesterday you really did not feel very strong effects.  You snort it and a few minutes later the lights go out.

What happend is that the blood levels of the buprenorphine had dropped overnight, and there wasn't any of the blockade effects that kept you alive yesterday.  At the same time, you increased the dose slightly, compounding the problem.

This simplified and melodramatic story might sound hokey, and you wouldn't be that dumb, but the problem is that this kind of thing does happen, and it is well documented that overdoses are common in this situation.  They key to staying safe when going off suboxone is to do it in a structured and controlled way, with the help of your doctor.  It can be done, but it takes time, and care, and you should not do it without help.