"60% growth" seems large, but there are almost 85,000,000 people in Germany, so we're talking about growing from ~0.03% to ~0.05% of the population here.
You're saying this now but they're on course to leave the left in the dust and are likely to compete with greens in terms of members and votes by election sunday next year.
Numbers on the far left are misleading as Die Linke is bleeding members to the BSW but the BSW is taking their sweet time handing out membership cards, double-checking every single applicant. Reportedly they have a backlog of at least 8000 applicants or such and at least 20k people generally interested. Wagenknecht is at least 70% tankie so it doesn't surprise that they're doing a vanguard thing.
Since not everyone is expected to become member of a political party, a more apt comparison would be to membership numbers (and changes in those) for other German parties.
People see a lot of immigrants coming into their area, and taking jobs. It’s not hard to see how people would draw a connection between the above statement and connect it to “that’s why I couldn’t get a job”. There’s a natural push back there.
Some companies see the influx of immigrants and realize they can get cheaper labor, and those that don’t get the job (the ones who already lived in that area) get disgruntled. It’s easy to talk some of them into joining a side that pushes back on immigration.
One of the things the far right looks at is immigration.
The left typically asks for bigger government and more social welfare programs (more taxes).
The right is typically smaller government and less social welfare programs (less taxes).
At a time when we are drowning in debt and can’t afford to buy food, have less taxes and more money in our pocket is an enticing idea. Easy to pull people to the right.
The worse the above problems get, the further to the right people go.
The further to the right they go, the less they hear from the left. Now they only see and hear one side. It becomes a vicious cycle at that point.