In the past, possibly decades ago, feminists usually had little to say about men. They almost always discussed women, and although any person has the right and the ability to discuss the soundness and reasoning of any argument or perspective, it might have been at least "emotionally understandable" (albeit never a binding restriction) when feminist women didn't like hearing men commenting on their theories, arguments, conclusions or hypotheses, when these may have had errors and men wanted to point it out.

However, in the past decade or so feminism moved from discussing primarily laws and labor, to a third domain: the intimate relations between the sexes. This inevitably caused feminism to start discussing men: men themselves, and men in the relations with women.

Feminism tells men what drives them, what bothers them, what's good for them, what manhood is, and what men need and don't need, and, in all matters related to relations, feminism attempts to analyze the human reality and draws conclusions that it then enforces on men, either directly or by driving women to follow its decrees and conclusions. This means that now and for several years, if the soundness or reasoning of feminist inferences are erroneous, this is not an error only about women - but about men, about their lives, and about things that should be done to men. Once feminism attempts to describe, explain or act upon: men, if it makes mistakes then correcting it or criticizing it is not "mansplaining", "men explaining to women on their private domain of womanhood" - feminists have crossed the field to explain to men what they are and to draft rules for them, and from that point onward men's criticism of feminism becomes a human right and often - if feminism becomes harmful to men - a moral obligation.

You cannot enact a theory about part of humanity, with conclusions that rule the lives of that part, but then not allow them to have complete freedom in criticizing it - this is called control and oppression.

Edit: I should also point out that if a man declares himself a feminist and by that accepts that his freedom of thought is confined to accepting with no judgment whatever feminist women say about him (otherwise he would be "mansplaining"), then him repeating their preaching without ever being allowed by feminists to employ autonomous judgment does not make a feminist conclusion "men's", just because a male was directed to utter it for feminists (and, needless to say, when that feminist conclusion or belief is false it remains false regardless of who was made to utter it).