Reasons why AB de Villiers retired sooner than expected 

#2 South Africa's Test calendar for 2018-19 international cricket season

South Africa has no high-profile Test series in next one year

Generally, most players prefer ending their limited overs career earlier than expected so that they can continue playing Test cricket for few more years. But AB decided to say goodbye at once. There may be two reasons for it, one being his lack of interest in playing the longest format of the game especially in the latter half of his career, which is evident by him staying away from low-profile series although, most of it was because of injuries.

And the other reason would be the lack of high-profile series for South Africa in the current season. Post AB's retirement, Sri Lanka hosted South Africa for a two-match Test series which SA lost 0-2. The next Test series for South Africa is against Pakistan in December at home where they will play 3 match series followed by Sri Lanka's return trip in February for two games.

In all likelihood, both the series will be one-sided with respect to South Africa and doesn't carry much significance in the cricketing world. This not so exciting Test calender might have influenced AB to call it a day at least a year before the expected time.