By 1903, Queen became aware that she is not heartily welcomed by people so she demanded wanted divorce from the King; I don't remember where I read that, but there were speculations at the Court that by early June of 1903, Queen Draga would have been deported from Kingdom of Serbia to Hungary or somewhere else.
Hearing that the Queen would leave Serbia, the officers allegedly made an agreement to kill them before Queen leave country. (This is just a speculation)
Also, the Danish journalist arrived to Belgrade (At cca. 10 a.m.) the same day the pair was murdered in order to interview King Alexander but it was too late, the pair was killed several hours before. (At approximately 3.30 a.m.).
But what fascinates me, how is it possible that both King or Queen wasn't informed about the possible revolution? Everyone in Belgrade talked about that at that time. Even in ate summer of 1901 the officers wanted to kill the pair.