The PS3 will change resolution for some things, depending on whether or not you allowed different settings in your configuration. 360 always scales to the same output resolution. It's very well possible that this resolution change is sometimes broadcast to the tv even though there isn't an actual resolution change. I think it depends on the TV how fast this is handled - on my PS3/TV combo it's rarely more than a second, and on my LCD screen I think it's even faster (probably because it doesn't have to resync sound).
Come to think of it, maybe sound over HDMI is related to it as well - if your BluRay supports a certain different audio output it may change that over HDMI and the TV may react to that. I don't know if anything like that would ever take place on the 360 considering the much more limited audio output options there, and I'm not sure that the 360 would change native output for things like that anyway.
Is that explanation possible? You could check what the audio playback mode is in your BluRay for instance by showing display info during playback, so you can see what audio mode is being used, and also check what different audio modes you've allowed in your audio settings.