Sounds to me like there was some work permit issues, Arsenal seemed interested and some reports that he got a work permit. But if he had got one, why did Arsenal not get him and why are we loaning to Anderlecht. Going to Anderlecht for a year would make a lot of sense if it allows him to get citizenship in Belgium.
Anderlecht seemed very keen, but knew they would be outbid on wages, it makes the development curve a little less steep and keeps him playing, developing. But he has only played 1 international friendly at start of the month, so would probably need special dispensation to get work permit. Another year in Belgium and it's no longer an issue.
Here's a translation from a Belgian site.
Everton snatches talented Henry Onyekuru ... and lends it to Anderlecht
It was one of the sagas of the summer market, it has just found its epilogue. According to our information, Henry Onyekuru , followed by Arsenal and West Ham in particular, will engage for five seasons with Everton, the English club of Romelu Lukaku and Kevin Mirallas . In the wake, the fast forward 20-year-old is loaned to the Sporting Anderlecht for a season. Henry has completely revealed this season with Eupen, scoring 22 goals.
The amount of the transfer is estimated at about 8 million euros, requested by the German-speaking club. The deal was facilitated by good relations between Toffees' management and Mogi Bayat, who was given " an exclusive mandate to negotiate on behalf of KAS Eupen ". The influential agent is also in excellent terms with the Anderlecht Sporting.