What causes a red wine to go through a "dumb phase"?

Hello there! I'm Dr. Vinifera, but you can call me Vinny. Ask me your toughest wine questions, from the fine points of etiquette to the science of winemaking. And don't worry, I'm no wine snob—you can also ask me those "dumb questions" you're too embarrased to ask your wine geek friends! I hope you find my answers educational, empowering and even amusing. And don't forget to check out my most asked questions and my full archives for all my Q&A classics.
Dear Dr. Vinny
What causes a red wine to go through a "dumb phase"?
—Frank, Portland
Dear Frank,
Lest anyone think we’re insulting wines, a wine is said to be going through a “dumb phase” when it is temporarily—and perhaps unexpectedly—“closed” or “inexpressive” as it ages. It can happen when a red wine transitions from youth to maturity: The wine sometimes goes through an awkward period where it isn’t showing well, and the flavors seem muted. That could, of course, be an ominous sign that the wine is in decline. Or it’s temporary, and the wine could bounce back and become better than ever. As you might be gathering, it’s unpredictable, and I should be clear that not every wine goes through a dumb phase.
There’s no known explanation for a dumb phase, and it can last for months or years. What’s interesting is that this is a contextual term, so unless you’re familiar with the wine, you don’t really know if the wine is going through this phase, or simply not to your liking. If you had an unlimited supply of an ageworthy wine and a cellar to store it, and every day you pulled a bottle from the cellar and served it at the perfect temperature, in a proper wineglass, and described it—you’d be able to track its progress and its dumb phase(s). Hence, I mostly hear winemakers, who are most familiar with their wines, use this phrase. Decanting doesn’t help; the only thing that can cure it is time.
—Dr. Vinny