If you don't defend ranking points, that's what happens. Murray used to be in the top 4 on the back of quite a few 'minor' tournaments won. Then he was second on the back of two Grand Slams, giving him 4,000.
He's won fewer minor ones this year as he's been injured/resting. You need to get to Semis or Finals of majors - he's got 360 for the US Open, as against 2,000 last year. Looks like he got 750 for the Olympics too. So that's 2,000 dropped in a few weeks.