I think the main part of your complaint here is not the quick grouping problem, or that your unintended HPF activations were caused by grouping (you say you didn't use it in this case).
However, I can say that at one point I was having tremendous problems with Quadcurve EQ's doing things by themselves. Sometimes I would load a project and every track would have this gigantic boost of 16dB at the same frequency on every track. Or, like your case, every track would have the high pass filter engaged at a very high cutoff point (thereby taking all of the lows and mids out of the mix). The problem would always seem to happen during the loading of the project - at least, it wouldn't be like that during my final last listen of the project the previous night, before saving and going to bed. And it would ruin hours of mixing. I did report it to Cakewalk but I didn't ever get an explanation, and the problem seems to have gone away on its own over time.
However, this is just the tip of the iceberg of plugin settings changing by themselves in Sonar. I used to have a huge problem with Native Instruments and Waves plugins being reset to default or changed by Sonar upon loading projects. Those problems were apparently fixed at the time, but the problem with Waves plugins has returned recently with a vengeance. I make extensive use of the Q10 EQ and the SSL Channel, and both are having their settings reset to default on project load. I'm not the only one too. I know these aren't ProChannel modules, but the thing they have in common is that their parameters are exposed to Sonar, and I believe that something in Sonar has access to these parameters when it shouldn't, and is firing off erroneous messages to them.
In fact I recently saw an example of this happening before my eyes, and was able to capture it happening. It happens in one of the affected plugins (in this case Q10) when you adjust a parameter while the ProChannel or FX bin which hosts the plugin is turned off. Upon turning the ProChannel or FX Bin on, the parameter receives a stray message and is either changed or reset. Here's an example:
Watch how the band I adjusted gets another "adjustment" when I turn on the ProChannel.
And here's the same thing happening when turning on an FX Bin:
So while this is a little different to your case, it does serve to illustrate that something is causing illicit messages to be sent to plugins without the intent of the user. I consider this to be a serious problem in a DAW - it can destroy hours of work just like that. In the project I'm working in now, I've had to save over 50 presets to my hard drive and every time I load the project, I have to go through the tracks and see which EQ's have been vandalized, and load their presets up. It doesn't happen consistently - sometimes they're reset, sometimes they're not.
To be honest I don't think this is a problem that is strictly limited to Sonar. I've read other reports of plugin resets happening in other DAW's - just yesterday I saw a guy on Reddit report that the Massive synth was having all of its settings reset every time he loaded a project in Ableton.
I know absolutely nothing about the coding side of VST's, but my guess is that there is something in the VST standard - call it a "back door" if you like - which leaves plugins vulnerable to "attack" from things beyond the user's control. And that DAW manufacturers need to do more to plug these holes and ensure that nothing can get to a plugin's settings except things that have been specifically requested by the user.