zulemara is correct. A guest was looking at the castle before the fireworks show, saw smoke coming out an exhaust fan from Cinderella's restaurant and called 911. Didn't inform a CM, just called saying the castle was on fire.
Of course Disney took it seriously (who wants a fire at the castle?) and dispatched Reedy Creek. The fireworks went on because the dispatch just said "fire" and either didn't give exact location or was misheard about where. Halfway through the show, Reedy Creek appeared and told them they had to reach the Castle. Castle=fireworks fallout zone and so the fireworks were immediately stopped. Once Reedy Creek gave the all clear, the fireworks started again.
Sorry it ruined your experience but at least now you know why.