Likely wanted to get/keep a contract for providing communication services when cell towers may be down. I think the specific issue that brought USAID's eye was when he disabled the satellites during a critical event.
Yup and probably cost lives by preventing aid from getting to where it needed to go. USAID signed a contract with starlink, Musk didn't keep his side off the bargain. Thus, investigation.
Despite the headline USAID wasn't investigating Musk or Starlink. It was investigating how Ukraine was using the Starlink terminals that USAID gave them and how USAID itself was overseeing Ukraine's use.
Basically the headline is completely misrepresenting what was happening.