You don't say what Avast program you are using, just wonder if it has the Avast Firewall component ?
If so do you have the Firewall set to Private or Public network mode ?
That said why it would only alert on one and not the others (but not knowing what they are) is strange.
My apologies, I am using Avast Premium Security. I do not have any of Avast Firewall components installed.
Anyway, I have 3 nvidia shield tv media streaming devices. They are all connected to my network with ethernet (not WiFi). The only differences in their configurations may be that they have different apps installed on them (ie, they may all have netflix, but only 2 may have hulu, etc.). Otherwise, all other settings are basically the same. So I found it very odd that one of them would be doing a "bruteforce" attack over SMB protocol.
And as strangely as the alerts started, they also just stopped. There has not been any more alerts since yesterday morning.
I opened support case with both AVAST and NVIDIA. I have not heard anything back from AVAST yet. I need to respond to NVIDIA after 24 - 48 hours to let them know if I am still getting the alerts.
Also, I did restart the SHIELD TV device that was generating the alerts. If I had to guess, these were maybe false positive alerts.