Praise be to Allaah.
This was mentioned by Muhammad ibn Umar al-Waaqidi, and was also narrated by al-Haakim in al-Mustadrak (4/6-7) and by Ibn Sad in al-Tabaqaat (8/76-77), and in Siyar Alaam al-Nubalaa (2/192) and in other sources.
It seems and Allaah knows best that she did not want her burial to be delayed, as it was reported that she died during the night on the 17th of Ramadaan, after Witr. Or it may have been because that was more concealing for her, or perhaps during her lifetime, especially towards the end of it, there appeared an idea that it was makrooh to bury people at night, and she wanted to make the correct Islamic ruling known. Or it may have been for some other reason. In general it is permissible to bury a person at night if there is a reason to do so. And Allaah knows best.