It may be a combination of that along with the snapshot and mount of VMs running on that NFS datastore to the VDP appliance. But in all honesty, you should be getting away from VDP as quickly as possible and on to something more capable and robust. This may be a lost cause trying to remedy given that VDP has been EoL for some time now.
Never used NFS storage for vDP to use (or live on). When I was using vDP, it had it's own datastore (and LUN) to consume. That was mainly so that it didn't effect any of the VMs that it was backing up. Also, I was using EqualLogic SAN arrays (a pair of PS6210X models) connecting via iSCSI (over dual active 10Gb connections per array). While there were issues with vDP, they weren't related to the storage it was using.
IF you're going to continue to use vDP, along with the NFS setup, then you need to make sure you have plenty of space available for it. You could, also, contact VMware support over this issue. I wouldn't expect them to offer much to help since there is already a replacement product available (if you're going to keep with something from VMware). Or you should seriously look to get a solid B&R solution in place. There are at least several solid offerings that won't break your budget. Well, unless your company is stupid cheap (like the one where I was using vDP).