Greggs' (LSE: GRG) shares – which have struggled for a long time now – have recently experienced a sharp move. Had an investor bought £5,000 worth of shares a month ago, that money would now be worth about £6,200. Why did Greggs shares pop? The reason the shares have popped is that H1 results, posted on 29 July, were solid. It's worth pointing out that Greggs' shares have been heavily shorted recently (meaning investors have been betting against them).