Why do Germans abandon key defence points

[Greyshaft]

Last turn the Germans had units in Budapest. They have now removed them and given me a clear run into the city. The same thing happened at Riga where the Germans abandoned Riga and moved their whole defensive line north into Courland. In both cases I had a strong unit there but the Germans could have made a fight out of it. Why do they give up key points?

[doomtrader]

AI usually moves units from one location to another if:
decides to attack
founds an opportunity to destroy a weak enemy unit or capture another city
founds that other more important town is in danger
decides that the front line in other place needs to be patched soon.