I think a crossed market is treated as a fault condition, something that requires some back-channel attention to straighten out. Given the gale that passed through Wall St today there might not have been much attention to spare.
"Those suffering the most are Nasdaq market makers who execute retail order flow and retail investors who may not receive the best price. This can cause auto-execution systems used by market makers to halt. If the bid and offer are the same, these systems assume the quote is stale or incorrect. If Nasdaq market makers auto-execution systems shut down, the market makers have to manually execute the orders which significantly slows trading" http://www.financetech.com/featured/showArticle.jhtml?articleID=14702074
This long article holds that the root of the problem is we have now competing pools of liquidity that are not as closely coupled as they might be. It's five years old but I suppose the color added still applies.