I don't believe it's cursed, but it has just been unfortunate in that said month looking at the chart , it's just a superstition believe and people always have what to say or believe and this is one of it ,if Bitcoin mostly experience a decrease in the month of September,if you watch very well it may not be always,but people are entitled to their opinion, but that doesn't necessarily mean it is cursed ,and if history decide to repeat itself,it's not guarantee that the month is cursed .
September is the start of the last quarter of the year, and that's where most people begin to take profits after having a profitable run. September is not cursed because it's not always a bearish month, take for instance in 2024 where it had the strongest run with a significant increase, many made tangible profits from it.
It's merely a historic sentiment as there is no single cause for depreciation in the price of bitcoin during this month, it's just the start of the month so there is no rush to jump into conclusions.