What is the logic behind raising the 99%?

There will always be a 1% and a 99%: that's the very nature of mathematics. So, framing the problem of income inequality in those terms, or even in terms of "equality" is ill-designed. The problem isn't that there is a range of wealth it's that the wealth is concentrated at the top, which puts a strangle-hold on the economy. Rich people spend relatively little of their money: they invest it or save it, thereby removing it from the economy. That means there is less absolute money for the non-uber-rich. If 99% of the people in the economy do not have enough money to buy goods and services, the companies that provide those goods and services will not survive. When those companies close down, jobs go with them. When the jobs go, the pay goes. If the cycle continues too far, the majority of people are unable to support themselves. The economy doesn't grow, and rich people can't make more money. So, while the affects are most devastating to the middle-class, rich people are negatively-impacted by a poor economy. It is in everyone's interest (financially, socially and morally) for there to be a strong middle class. It is the engine that drives the economy and keeps all else afloat.

History.

When too MUCH of a nation's "wealth" becomes concentrated in a (relatively) small group of hands those folks become so powerful they basically control the population....it was a rebellion AGAINST that sort of "royalty" that resulted in the founding of your nation....and because even the US model of a republic was not 100% effective against that "inter-generational concentration" of wealth (and consequently power) your Congress had to pass the Sherman Anti-Trust Act in 1890 to prevent folks like the Mellons, the Vanderbilts, & the Rockerfellers from becoming the de facto "un-elected government" of the USA...

Who "decides" how much power & wealth is "too much"? In the USA, we still have democratic elections, so it is the population that do...

You are an excellent example of a (rather prevalent) type of new-ish "binary American thinking" in which something must either be "all good" or "all bad", and that's not how the REAL world works at all! The irony in that is that Orwell's pigs practiced EXACTLY the same sort of "binary thinking" when they decided "two legs good, four legs bad"!

"what is the logic behind that? "

It is an emotional appeal to the voter's envy, nothing more than that.

A viable middle class is critical to a successful market-based economy. Buying stuff drives the economic engine. Investing wealth merely allows producers to operate fulfilling demand. Concentrating too much power (ability to contribute and actively participate in the political process) in the hands of a few risks autocracy.

Visibly rewarding successful high achievers promotes further success. Blanket redistribution of wealth depresses achievement.