Basically, the sentence stands like this in my textbook, without further context. The exercise instructs to read each one of the 20 sentences (of which this is just one) and mark wether this happened around me lately or not.
Maybe it means "you fell"/"you became falling"? I also can't really make sense of the function of そうに here. I interprete it in the way of 楽しそうに遊びます (=playing cheerfully), but I can't think of a way to implement this in meaningful way here. Also, should ...そうになる be a set phrase, then I don't know about it or I can't find it in my texbook.