The MM's, or some of them have logged off the stock for the evening. More than likely to back up systems etc. When they do, the bid and ask will reflect whomever is sill logged on. Probably an open retail order, but that would be just a guess as you or I would never know.
Sometimes it will show 0 ask and 0 bid as well. The spread will tighten up in the morning before open. It means absolutely nothing though as to the direction of the stock.
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