any backup software would backup VMDK of VMs, so it creates Snapshot first, then locks the vmdk file, backups the data then deletes snapshots.
after backup, backup process might get terminated on veeam or ESXi might not respond to the backup due to its high resource utilization.
if you see the VM files on the data store then you would find more then expected vmdk files to be. this vmdk files will be locked by veeam try disk consolidation.
If you are using veeam virtual appliance, then go to veeam virtual appliance 'edit settings' and check how many hard disk are given. remove all the disks attached and try consolidating the snapshots. 