It's too early to tell that bitcoin is rallying solely based these couple of days of increase. Assuming that bitcoin is rallying towards through out the end of the year, it might be due to the feeling of being optimistic for year 2019. The bitcoin ETF is expected to be approved on February. This is a good trigger thing for bull run.
BTC ETF is one of the biggest reasons. Also, don't forget Bakkt platform's release. Also, if you invest your money into BTC right now and sell it next year, you would pay less tax bucks (long-term capital gain tax in USA).