Unlike his first death, William Afton escaped the Fazbear Fright’s fire through more realistic means. William realized as he was burning that there must be an exit close by because he had a brief encounter with Micheal just before the place went down in flames.  Thus, if Micheal made it out alive after starting the fire, there must be some kind of exit in the vicinity. And so, Springtrap gambled on the assumption that his son must’ve escaped in one piece and sought out an exit, which he eventually did find.