I've just woke up a bit confused... are we poaching eggs in wine and beer now.....????? Laughed out loud, pfr, I really did.

To attempt to address the original premise i.e. strictly about bee... No, let's be a bit more precise, fermented malted barley (mostly) with hops added for flavouring & preservative properties. Beer & lager are fundamentally different drinks. For one, the yeast ferment at the top, the other ferments at the bottom. Yes, that matters (I'm not entirely sure why).

More to the point, lager is brewed to be drunk chilled, around 4C, whilst the jokes about warm British beer are true up to a point - beer is brewed to be drunk at 'room' temperature, which actually means cellar temperature - 10-14C. The usual disparaging remark I make around this time is something like, beer is brewed so you can taste it, whilst lager is brewed so you can't! Which is completely unfair on lager. It should be properly chilled; if it tastes horrible warm, it's because it's not supposed be drunk then!

The problem with your question is you are American. Carlsberg are a Danish company. They make a big deal of that in UK adverts. Not that our Carlsberg is Danish, any more than your's is. Brand beers are brewed under license locally across the globe. Budweiser was a Czech lager originally. Etc. What you get is not the necessarily the same drink (not even if it's labelled "Export"). I cannot give you a scientific reason why, but beer & lager are different drinks with different cellaring requirements. What the US, where you are, describes as 'beer', in most cases as far as I can tell, is actually lager (& possibly inferior lager, as much as UK lager mostly is). But, given the difference between beer & lager, they will likely behave differently at the same temperature. And any drink will itself behave differently at different temperatures.

And finally a direct answer to your question! I rarely drink lager (only if I know it's German / Czech brewed), as I prefer dark beers; porters, stouts, browns, etc. And, obviously, I wouldn't drink it warm. However... I occasionally have the same problem of foaming with an out-of-date beer, or with a beer such as Leffe, which is corked & wired, rather than capped (and in larger, thicker bottles too). It's pressure, I think. Water is incompressible, but gas most certainly isn't! The Leffe is obviously bottled under higher pressure, or they wouldn't do it the way that they do (same as sparkling wine compared with still). An out-of-date beer may have undergone unplanned secondary fermentation, which increases the gas pressure within the bottle. One of the reasons I'm not fond of lager is I find it rather fizzy i.e. gassy, implying it is also bottled under greater pressure than a beer. If the contents of the bottle are 10-20-30 degrees (whatever the scale) higher than intended... The gas wants to occupy more volume, but can't, so the pressure increases. The moment the pressure is released by opening, it therefore expands more rapidly than the makers intended & hey Presto! Foaming!! I reckon that's almost certainly the correct explanation, Michael.

If you want to prove it by experiment, stick a bottle in the oven & see how high you have to go before the bottle shatters or, possibly more likely, the cap shoots off! ;-)