@Musashi
Much of that growth is owed to Eastern German voters, the reasons being structural. East Germany has lost 20% of its population since 1990, the brain drain creating somewhat of an echo chamber. The average East German is above median age, tends to be xenophobic and idealises the Socialist past. The AfD successfully snatched the image of being the "natural" representative of East Germany from its former owner, Die Linke (the successor of the regime party SED, which is on the verge of collapsing into obscurity at this point).
Sadly, it can't be denied that East Germans are predominantly against the government's pro-Ukraine policies. My personal theory is they subconsciously feel like they owe Russia. For fourty years they'd been told by the Russians that West Germany was the Nazi successor state and that they didn't bear the blame. The Russians consciously nourished their national pride and self-image as being very distinct from the people in West, so as to eradicate all wishes for reunification from their thoughts. They also made them hate America.
That's what's showing in the polls, at least to some extent.
But AfD has also been able to scoop up many protest voters. I'm not going to pretend that every decision taken by this administration has been bad (I'm certainly agreeing with their foreign policies and defence build-up, for instance), but their social and environmental policies are a clusterfuck of epic proportions. They're woke as hell, dangerously elitist and appear contemptuous of the wants and needs of the general population. They want to give children as young as 14 the right to change their gender, for example. They want to de-criminalise hit-and-runs.
Most notably, they've tried pushing through a law that would make it illegal within two years to operate gas or oil heating, forcing all home owners to buy an electric heat pump instead. Only the elderly or the poor were to be exempted. Nevermind the blatant injustice and infringing on people's property rights, their communication was a non-stop charade. The bill's chief author had to step down over a nepotism affair, the text had to be re-written over and over again until no one knew what it meant, and it was just the other day halted by the courts.
This administration gives of the impression of a bunch of bumbling fools who'll ride roughshod over this country no matter what to implement their woke agenda. AfD represents the antithesis to that agenda, so naturally people are flocking to them. Especially since the moderate right has been so thoroughly compromised. The CDU can't fill the role of opposition leader as they're seen as a weak compromise between the government and AfD … and the FDP is in the government. So these polls are also somewhat of a vote of no confidence.
The AfD's place in the polls is not sustainable, though. Even their honorary chairman admitted as much in a recent interview, saying that "our strength is merely the weakness of our rivals". He advised his party not to make a bid for chancellorship, as it wouldn't come to pass anyway.