The violent and abusive people who are issuing the threats.
Well yes 
Violent and abusive people are getting angry and so sending abuse and threats. Saying that anger was incited was in no way minimising.
“The definition is shifting” isn’t good enough. That’s not what is happening- the definition is being taken. It is deeply harmful to lesbians who are already a sidelined and oppressed minority.
Trans-inclusive lesbians continuing to identify as lesbians has meant a shift in the way people perceive lesbianism. That doesn't mean the definition is being 'taken', it means it's being opened up a little to include people and relationships who best suit the term.
Somebody calling her trans girlfriend her girlfriend, continuing to operate under the label 'lesbian', and just living her life in happiness is not "taking" the definition from you. I'm not sure if I've missed something but I don't think anybody - and that includes violent, selfish trans people - is campaigning to have lesbian relationships in which only females are involved no longer being counted under the umbrella. Then the definition would be taken.
Trans identifying people have been around for ever. It’s only very recently that transactivists have begun demanding that we, women, accept that they are literal women. That lesbians can have cocks.
Because it's possibly the first opportunity that - in the Western world - trans people are in the position to request things like that. It's not odd that with more rights and acceptance, they will begin to speak up on smaller issues affecting them.
The overwhelming majority of these transactivists are white, educated, youngish men who have recently begun identifying as trans.
Do you have stats for this?
Not in a nasty way, I've just never heard this before. I did read a very interesting study that Caucasian families might be the most accepting, which could have something to do with it.
It’s an absolute shitshow and as per usual, it’s women who are expected to move over. And lesbians, doubly so.
Trans men/boys like to be referred to as men/boys.
There are trans inclusive men.
We hear a lot more about trans women, but it doesn't mean that women are the only ones affected.
So what is it based on?
As a cis person, I'm entirely unqualified to answer. I can't tell you what it's like to be trans. You'd be better off going online and reading about trans experiences/issues than asking me.
But, it's based on some kind of psychological conviction or understanding that you are a particular gender. Just as I would not stop being a woman if I threw out all my dresses and declared eternal hatred for the colour pink, neither would a trans woman. It's not based on stereotypes.