When they went
upstairs to the crowded common room, it was to find Fred and George had set off
half a dozen Dungbombs in a fit of end-of-term high spirits.
背景是Harry，Hermione还有Ron（哈利波特的主人公）从别的地方回来。
我在Google上只找到了一个相关内容：
https://forum.wordreference.com/threads/it-was-to-find-that.2669737/
大概理解成They went upstairs to find或者and find....，表示结果或者展现出惊讶的语气。但是这个结构我从来没见过，网上也没找到过类似的。
希望网站的老师专家高手能帮我解答一下，谢谢~
