1. You tweet too much.
A tweet a minute? A bit much. Overload. Even celebrities that get paid per tweet (yes, some do, and they get some serious cash from it) don't usually tweet that much. Even if something is super-exciting and you tweet incessantly for 5 minutes, give it a rest for a while. Let your followers come down from their tweet overdose before you tweet again.
2. You tweet about boring topics.
Now one person's boring is not another person's boring. Bert on "Sesame Street" is into pigeons, bottle caps, and oatmeal. And we don't begrudge him that. It's part of his charm. However, if Bert was only tweeting about oatmeal (for example, the pros and cons of rolled vs. steel cut) all day, he probably would drop some followers. Granted, there are oatmeal aficionados out there, and they would appreciate Bert's tweets. But oatmealophiles are way outnumbered.