Why was Fairy Tail disbanded? Why suddenly the guild members decided they go each on his way?

As Lucy describeds it, no one opposed the decision of just close the guild.
Your question can be answered only in big spoiler as it is revealed in the next chapters. So...

SPOILER ALERT


   Makarov disbanded Fairy Tail so that its members would not get caught in a fight with the Alvarez Empire. After he disbanded the guild, he went to the Alvarez Empire where it was revealed that the Emperor is none other than Zeref himself. Zeref wanted the Lumen Histoire, which is Mavis' body (or more precisely, her ability to use imagination) in order to fight Acnologia. Lumen Histoire is hidden in the basement of Fairy Tail guild building. If the guild is not disbanded, then the whole members would get killed in the fight due to the sheer difference in power between them. At least this was what Makarov thought.