Why are we getting error " I/O is frozen on database CRM_DB" ?

Application API's not responding while SQL Server backup is running.

we are using storagecraft - a tool to back up the DB in every 15 minutes in NAS.
https://www.storagecraft.com/products/shadowprotect

10/20/2018 11:46  --- I/O is frozen on database CRM_DB. No user action is required. However, if I/O is not resumed promptly, you could cancel the backup.
10/20/2018 11:46  --- I/O was resumed on database  CRM_DB. No. No user action is required.
10/20/2018 11:46  --- Database backed up. Database: CRM_DBcreation date(time): 2016/01/06(22:09:09),
                                       pages dumped: 1285028, first LSN: XXXXXXXXXXXXX,
                                       last LSN: XXXXXXXXX, number of dump devices: 1,
                                       device information: (FILE=1, TYPE=VIRTUAL_DEVICE: {'{39}3'}).
                                       his is an informational message only. No user action is required.

It's taking 8 database backups including master, model & msdb

We are thinking this may be one of the reasons for this issue. Is it right? Anyone facing this issue? How to correct it?

OS: Windows server 2012. SQL server 2012.  Server is VM