Casdon
Trump can’t bear the spotlight on anybody else, he even does down and humiliates his own deputy and supporters. It’s one of his most unattractive traits.
Starmer was probably about to welcome the participants to the country (his country, where he is the host, and which has provided the venue they are meeting in and the security that ensures that none of them is assassinated by zealots while they meet to discuss the details of this critical ceasefire and a potential end to hostilities) That would have been a short polite introduction, and he would have stepped back while Trump went on to chair the meeting and get the kudos. If they have a written agenda, it would not surprise me if that was "Item One - UK PM welcomes delegates to UK" followed by "Item Two - US President opens discussion"
Trump has the chair - which itself might well have been reserved for the PM of the country in which they are meeting, but would have put DT's nose so far out of joint that he could have abandoned the whole project in a strop. However, the niceties of allowing the host a couple of minutes in the spotlight would be alien to him.