Is it true that the market usually rises before the end of the year?
If so, why do you think it happens?
I spoke with few people that already said that in the last 5 years the market became really bulish
This is a behavior that we have seen out of bitcoin and yet I have not seen an explanation that tell us why this happens in a satisfactory manner, but you need to remember that just because that has happened in the last years it doesn't mean that is going to happen again, if you invest money in bitcoin thinking this is going to happen again only to find out that you made a mistake then not only you are going to lose money you could lose confidence in bitcoin as well.
Market trends often seem to form out of nothing, the first few years may have just been a random bullish period and now people jump on it and assume the same will happen again. If they continue to do that then the price will only rise in Q4. As for a reason there doesn't seem to be much of a logical one, perhaps just a seasonal thing that people tend to have a little more spare money around that time of year, in some countries you receive double pay in the month of December.