posible reasons for invoice split

Hi,

In development client i had created one test sales cycle.in this i found that against one order system had created 2 billing documents.

i cross checked and confirmed the out of 7 line items system had created one billing doc for 2 line items and 1 billing doc for 5 lne items.all this materials are belongs to one division only.

can some one suggest me posible reasons for invoice split.