Banks' trading desks ended 2018 with a whimper – and nowhere as much as at Morgan Stanley.

The New York-based firm had the worst bond trading performance among the big five Wall Street investment banks, a precipitous 30 percent decline to $564 million. It was one of the most meager hauls since Morgan Stanley revamped the struggling business at the end of 2015, prompting a flurry of questions from analysts.

"I'm struggling with the quarter understanding exactly what happened," Mike Mayo, a veteran bank analyst at Wells Fargo, said Thursday during the call. "On the one hand, we could say, 'Oh, this is Morgan Stanley acting conservatively, scrubbing the balance sheet, etc. On the other hand, it could be the Morgan Stanley of old where you had hiccups in fixed income like the second half of 2015 or 2013 or during the financial crisis."

After the 2008 crisis reordered the banking world, Wall Street firms sought to reduce the volatility of results across trading and advisory businesses. Goldman Sachs and Morgan Stanley, where trading still makes up a bigger slice of revenue than peers, have sought new sources of revenue in consumer banking or wealth management.

But the sharp declines in December showed that, while they've made progress, banks are still exposed to the whims of the market.