Hyper-V VMs are in saved state after backup

Hi, my master server is NBU 7.6.0.3. We are backing up VMs in a Hyper-V server. This is a single node Hyper-V server (Windows Server 2012, NBU 7.6.0.3) with a few VMs. Yesterday we found some of the VMs are placed into saved state after backup. 

My question is:

1) What is the reason of this? Is it because of resource limitations? 

2) If it is possbily because of resource limitations, what are the recommended resource requirements for NBU backup of Hyper-V VMs?

Thanks in advance.