Veeam will assign tasks based on multiple factors. Here’s what I found in the forums:

For each VM in the job, backup server classifies all backup proxies available to the job into 3 groups by access type they have to the VM's disks: SAN (1), hot add (2) and network (3). Proxy with least tasks currently assigned is picked from group 1, and if responding, VM processing task is assigned to this proxy. Otherwise, the next least busy proxy is selected from group 1. If all available proxies group 1 are already running max number of concurrent tasks, selection process switches to group 2 and repeats. Group 3 is only used if groups 1 and 2 have no responding proxies. For group 3, proxy subnet is also considered when picking the best proxy.

Based on your screenshot, it looks like both are using NBD. That means that at the time of proxy selection, the proxy that was selected had more free tasks than the other proxy. If you don’t want that proxy being used in that job, you can force it to use certain proxies here:
 

Also, check to see how many concurrent tasks you have set on the “VMware Backup Proxy”

Hey ​@hs08 
How many proxies have you deployed on your infrastructure? Are they virtual or physical? By default, the VBR server itself is a proxy. Veeam distributes the load between the proxies according to the location of the machines and the load of each one.

Network mode (nbd) is a mode that is very versatile in that it will work in any situation but it s very slow cause it uses the ESXi management interface.

hi ​@Tommy O'Shea 

it’s work after i set manually the proxy in job setting.

It is best practice to deploy proxy servers and disable the VBR server from being a proxy.  This way you don't have this happen.  Something to consider as well as deploying more proxies.

Hey there ​@hs08 -

Here are a few resources to help out with Proxies, and a KB for when using VMs as Proxies:

https://www.veeam.com/kb1184

https://bp.veeam.com/vbr/2_Design_Structures/D_Veeam_Components/D_backup_proxies/

Very good info imo. 

Best!