Why did my beer slowly froth up after removing from freezer?
Melting takes time. 
As your beer is thawing, temperature remains roughly constant. Then the beverage will slowly assume ambient temperature. 
Solubility of gases (e.g., $\ce{CO2}$) in common solvents usually decreases with warming. This is because the breaking of solvent$-$solvent interactions are not compensated by new interactions between solute and solvent. In other words, solubility of gases is often exothermic.




The bottom of the beer might have warmed more quickly, causing a buildup of carbon dioxide under a layer of still-frozen beer. Reaching some critical value for pressure, the top popped. According to Henry's law, immediately after even more gas bubbles are formed.

Effects due to positive thermal expansion of beer are probably rather small but these would only aggravate any overflow.



For further reading on similar subjects, see 


Does shaking increase the pressure inside a bottle of champagne?,
Plateau–Rayleigh instability.