Twitter is loosely based on probability theory. Just as the chance that a die will roll a six is equal every time it is rolled, so it is that the probability of you losing a follower is the same for every tweet you post.
This process has no memory. If you post five tweets without losing a follower, then the odds are not stacked up for a lost follower at the sixth tweet - the odds are the same as always, no change. And just as a die can be rolled for hundreds of times without a single six facing up, so you can tweet the longest of rants without a single unfollower.
And then you do one random retweet, and poff! Someone no longer has you in their life.
There doesn't seem to be any way to predict when this happens. The closest one gets is an estimate of the mean time to happen, which doesn't tell you all that much. "It will most likely maybe happen every x tweet, but it might also not happen" is not a paradigmatic statement of probabilistic determination. In fact, the word "uncertain" is more appropriate.
Strangely enough, this description does not seem to apply to gaining new followers. They don't have a mean time to happen; they only have good times to happen.
Strange, strange Twitter.