This is due to the load that is passed to each anchor. You might think that (for example) two anchors will receive 50% load each during a fall or simply when the climber weights the rope. But actually the amount of load an anchor receives depends on the angle of the rope that this is attached to.
Typically a belay with two anchor points forms a triangle
Anchor Anchor
* *
* *
* *
*
Attachment
point
The angle (at the attachment point) of this triangle dictates the amount of load that is passed to each anchor. If the angle is 60 degrees of less each anchor point receives 58% or the load placed on the attachment point. This is generally regarded as a safe amount.
As the angle increases beyond 60 degrees:
Anchor Anchor
* *
* *
* *
* *
* *
* *
*
Attachment
point
The load placed on each anchor also increases. At 90 degrees the load is 71%. At 120 Degrees it's 100%!!
So you should always keep the angle between anchors under 60 degrees.