According to Rosenberg, "There are tons of firms which have decided that, since they have no choice but to merge up, (they have to start) talking to potential buyers." Those who wait too long may be left to "almost literally die on the vine," he warns.

That makes it imperative for accountants within small firms to keep tabs on their employer's succession plans - even if you're not yet close to making partner.

Firms with three to 10 partners appear to be the most vulnerable, says Joel Sinkin principal at Accounting Transition Advisors LLC. Many such firms have several partners aged 55 to 65, and many also have great difficulty recruiting and developing staff who can eventually take over the firm. Whether a shop has youthful talent appears to be a key criterion for prospective buyers. Says Rosenberg: "They're interested in partners who are young enough to stay around for many years. And most of all, do they have young staff?"

When owners of a small CPA firm sell out, the buyer is often a regional or nationwide firm. About 90 percent of the top 100 accounting firms either recently completed a merger or are in talks that could lead to one.

Regional firms appear to be the most active. Those buyers "want to dominate their entire patch and are filling in the gaps like a jigsaw puzzle," California-based CPA consultant Chris Frederiksen told WebCPA's Stuart Kahan. Other motivations include "to get a larger presence in their home market, to get niche dominance (for example, not-for-profit or construction), to gain an expertise they don't have or embellish what they've got, to gain staff, and to improve profits." In contrast, the Big Four are relying more on organic growth, especially outside the U.S."