This is one of unique ways of expressing in Japanese , called "Gion-go"（擬音語） When you read japanese comics, you would notice that there are many words in frames such as ”どかん”、”ドン”、”タッタッ” etc. Japanese comic writers express sounds and situations in a scene using words like those. For example, どっかん or どかん are nomally used as sound of a great impact like explosion. I think you have seen expressions like them such as "CLASH!!" in American comics. And they express not only real sound, but also does the atomosphere in a scene.(It' called"Gitai-go)" For instance, "シーン" expresses silence. You might think it strange that Japaneses uses words to express silence. There are many Gion-go and Gtai-go in Japanese. So it may be interesting to look them up in a dictionary.