If my traditional friends will remember, we know that the planets Mars, Jupiter, and Saturn retrograde at the superior trine with the Sun. Yet recall that when Saturn stationed retrograde in March 2015, he stationed in the 5th degree of Sagittarius, while the Sun was in the 24th degree of Pisces. Is that a trine? Not via the Zodiac. So Saturn squared Sun in the zodiac, yet stationed? It bears considering.
Actually, this isn't the case.
The aspects to the Sun merely mark the beginning of the phases of the planets apparent cycle of movement.
a) The
square aspect to the Sun (90°) marks the beginning of the
First Station: this is when the planets begins loosing speed by a drastical amount regarding its average daily motion; eventually it will go "stationary" and then retrograde.
b) The
trine aspect to the Sun (120°) is called the
Acronical Rising: it begins after the trine, and it is concerned with the visibility of the planet after the Sun sets in the horizon; a planet in its acronical rising stage, will always be retrograding.
Now the thing is the ancient astronomers (and astrologers) never marked the "trine" as the period in which the planet goes retrograde, but rather state that a planet will always be retrograding after it trines the Sun. It has always been stated that the planet will retrograde once its velocity drops to 0°, and this has always been known to happen in between these 2 stages of the planetary movement.
In the example you provided of Saturn going retrograde before the trine, this is to be expected. If you notice the velocity of Saturn after its square with the Sun (around 22 February 2015), the apparent speed loss is much greater than before. This occurs during Saturn's entry into first station stage.
In essence, some astrologers have mention that the planets go retrograde after the trine with the Sun, but this is a mere indication that the planets will always be retrograding by this stage, but note this is not the "traditional" view on it.
To sum up: Aspects, are not really related to the beginning of the retrograde motion, they just mark the beginning of the phases