ReactJS: Why are the constructor called when setState changes the state

I'm a newbie to ReactJS and I have made an app where you can submit a name and email. The name and mail should be displayed in a list at the bottom of the page. It is displayed for a short period, but then the constructor gets called and clears the state and the list.

Why is the constructor called after the state change? I thought the constructor only runs once and then the render-method runs after setState() changes the state.

class App extends React.Component {
    constructor(props) {
        super(props);

        console.log("App constructor");

        this.state = {
          signedUpPeople: []
        };

        this.signUp = this.signUp.bind(this);
    }

    signUp(person) {
        this.setState({
          signedUpPeople: this.state.signedUpPeople.concat(person)
        });
    }

    render() {
        return (
          <div>
            <SignUpForm signUp={this.signUp} />
            <SignedUpList list={this.state.signedUpPeople} />
          </div>
        );
    }
}

class SignUpForm extends React.Component {
    constructor(props) {
        super(props);

        console.log("SignUpForm constructor");

        this.state = {
          name: "",
          email: ""
        };

        this.changeValue = this.changeValue.bind(this);
        this.onSubmitForm = this.onSubmitForm.bind(this);
    }

    changeValue(event) {
        const value = event.target.value;
        const name = event.target.name;

        this.setState({
          name: value
        });
    }

    onSubmitForm() {
        this.props.signUp(this.state);
        this.setState({
          name: "",
          email: ""
        });
    }

    render() {
        console.log("SignUpForm render");
        return (
          <div>
            <h2>Sign up</h2>
            <form onSubmit={this.onSubmitForm}>
              <label htmlFor="name">Name:</label>
              <input id="name" name="name" onChange={this.changeValue} />
              <br />
              <label htmlFor="email">E-mail:</label>
              <input id="email" name="name" onChange={this.changeValue} />
              <input type="submit" value="Sign up" />
            </form>
          </div>
        );
    }
}

class SignedUpList extends React.Component {
    render() {
        console.log("SignedUpList render");
        return (
          <div>
            <h2>Already signed up</h2>
            <ul>
              {this.props.list.map(({ name, email }, index) => (
                <li key={index}>
                  {name}, {email}
                </li>
              ))}
            </ul>
          </div>
        );
    }
}

ReactDOM.render(<App />, window.document.getElementById("app"));

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