Sooner or later, groups disband. It's the sad truth. In the early years of Kpop, groups would suffer from the 5-year-curse, which meant that once a group hit 5 years together, chances of them breaking up increased severely. Examples are H.O.T (1996-2001) and S.E.S (1997-2002), who both lasted exactly for 5 years before splitting up. Nowadays, this curse seems to have somewhat vanished, with groups like Big Bang, Super Junior, SNSD and SHINee having stuck together for way over 5 years and are still going strong. And of course, who could forget Shinhwa?
However, as a group approaches the 5 year mark (or even earlier), most members have their own individual activities and the group only comes together for the annual comeback and the year-end shows, so it's not so strange that maybe one or two members will release solo material, or even form a sub-unit.