Can doctors tell me what does tumor to grow so rapidly with high speed all the sudden?

Possible mutation: Tumors change. From your question it seems that you are implying that a likely benign tumor (as opposed to using the term cancer) was watched and grew slowly for a while. The it grew noticeably faster. This could suggest that the tumor had changed or mutated or progressed to a higher grade. It biological properties changed so that it appeared more aggressive.