Take AAPL as an example. There are two queues: Bid and ASK
BID: ....137.80, 137.90, 138.00
ASK: 138.00, 138.10, 138.20, ....
If some weird (by mistake of traders) order comes in to buy AAPL for 157, it will cut in and jump before 138.00 in the bid queue. This order could be filled for 157 or 138 by the system. If it is filled for 157, you will see a jump in the chart.
Of course, that selling guy is lucky. He intends to sell for 138, but he got 157. My orders from IB are often filled for better prices than I expected.
When I cover my short poistions, I often use a higher bid to cut in. For example, when APPL was traded for 138.85, I sent an order to cover for 139. Usually, I got filled under 139.