TL;DR there was a backdoor found in the XZ program. All major distros have been updated but it is recommended that you do a fresh install on systems that are exposed to the internet and that had the bad version of the program. Only upstream distros were affected.
Probably for the exact same reason this backdoor was introduced. Users complain about slow feature rollouts so (unpaid) devs (maintaining software in their spare time out of the kindness of their hearts) cut corners. In some situations that looks like bringing on a second maintainer without thorough vetting, in others it looks like importing upstream packages without thorough vetting.
Don’t blame the Termux devs here, blame the community that keeps pushing them to move faster.