Why is traffic to a smartphone causing traffic to drop on independent switches.

I know that subreddit is generally for enterprise networking discussion, but this once has really stumped me.

https://imgur.com/a/j2U5dwH

On a fairly small office network, we have been having the entire network drop intermittently (about 5 seconds at a time at 30 secondinterfals). Last night, I unplugged everything in the office, except for a few computer, a wireless AP, 3 24-port switches, and a VM that runs a web server, and tracked it to transferring files to a smartphone over wireless.

Files are stored on a ZFS array (NAS), and served to VM A that runs Nginx and a NextCloud instance.

Computer B can happily saturate its gigabit connection while downloading from VM A, without causing network outage. A laptop can download at gigabit speeds via the wireless router, from VM A. Everyone is happily pushing Gigabit traffic throughout the network.

• Whenever the Android smartphone is used to download files from VM A, the entire network drops, including communication between Computer B and Computer C, which are the only 2 devices I had connected to Switch C.
• The NAS can no longer reach the Router.
• Router doesn't lose connectivity to the Internet.
• Switch A is an unmanaged 24-port Netgear JGS524 switch.
• Switch B and C are unmanaged 24-port TP-Link switches.

What is going on with this? How can one smartphone take down the entire network? I even suspected that something weird was happening to power, and tested with 2 computers in a cubicle, uplinked to Switch A, and they also loss connectivity to each other for 5 seconds.