Tennis rankings work on a 12 month rolling period. So today's ranking are from all the points you accumulated over the past 12 months.
Federer gained 2000 points from winning the Australian open last year, but those points have dropped from his tally as it was over 12 months ago. Federer gained 360 points from his quarter final this year. So it's a net loss of 1440 points to his tally, hence the drop in the rankings.
At least you might see some other guys challenging within 18 months. This might be the last year of the real dominance for Djokovic and Nadal. You'd expect them to split the Grand Slams in 2018, only injury would tell me otherwise. However in 12 months you will have Tisipas and a few others like Coric challenging. Think Coric may break out at the US Open and make a huge run (just a gut feel)