The controller is likely to be throwing errors. Look in Help/Error Log.

I’ve never experienced this.

Errors related to music services, connectivity failures, etc trigger a popup. This activates the main controller window.

Thank you for the feedback @ratty!  I am indeed getting errors.  My large playlist is returning a lot of errors about improper encoding for some songs.  

This error is often caused by packet loss on the local network. To start with I'd suggest you simply restart everything, in case there's an IP conflict somewhere. If that doesn't help you should go looking for anything which might account for wireless interference. 

@ratty - Thanks again for your feedback.  I’ve rebooted all devices: modem, router, bridge, speakers, and laptops.  I’m still getting errors.  I’m thinking it may be a YouTube music issue.  All songs in the playlist being played when the errors occur are when playing songs from YouTube music.

If it’s limited to just one source, the “improper coding” error could indeed be just that: something going wrong at the server end or over the internet.

A local network glitch would be expected to affect all sources, with it more likely to occur on higher bandwidth content. YouTube streams at only modest bitrates.

EDIT: I just noticed your mention of “bridge”. The Sonos Bridge is now deprecated. It’s old tech and its power supply has a habit of losing voltage as it ages, giving rise to intermittent connectivity problems. 

Remove a Bridge from your Sonos system

I recommend that you assume BRIDGE is causing the issue until proven otherwise. If you have a voltmeter, the BRIDGE power supply should measure slightly over 5 Volts. Below 5 Volts you’ll experience intermittent, variable, and very hard do describe issues. If you cannot measure voltage, power down BRIDGE and wire another player if necessary.