It's an ethinic conflict that erupted a few months ago and has escalated since.

The conflict was triggered when the state High Court gave a verdict in favour of the Meiti community for a long standing demand for special previliges.

The Wikipedia article about the violence has a good overview.

Seconded, although it's not like gangs of criminals gang-raping innocent people is a phenomenon that demands a nuanced explanation. It's possible that they did it because they had guns, and they wanted to. There are places in India where criminal gangs operate freely.

This article attempts to provide some reasoning.

As for the neighbouring area, since it's mentioned near the end of this article, a related fact from wikipedia:

Notably, opium production in Myanmar is the world's second-largest source of opium after Afghanistan, producing some 25% of the world's opium, forming part of the Golden Triangle. While opium poppy cultivation in Myanmar had declined year-on-year since 2015, cultivation area increased by 33% totalling 40,100 hectares alongside an 88% increase in yield potential to 790 metric tonnes in 2022 according to latest data from the United Nations Office on Drugs and Crime (UNODC) Myanmar Opium Survey 2022[283] With that said, the United Nations Office on Drugs and Crime (UNODC) has also warned that opium production in Myanmar may rise again if the economic crunch brought on by COVID-19 and the country's February 1 military coup persists, with significant public health and security consequences for much of Asia

More often than not, ethnic disputes are just leverage used by people in power to achieve their goals.

Besides the brutality of mentioned in the OP, there have been tens of deaths in the area during the past few months.