Timestamp column being updated for all the update() on the data.

[sathiyad]

I have created a customized subscription model (not Laravel Cashier).
the Migration as below

public function up()
{
    Schema::create('subscriptions', function (Blueprint $table) {
        $table->id();
        $table->bigInteger('user_id')->unsigned();
        $table->foreign('user_id')->references('id')->on('users');
        $table->bigInteger('plan_id')->unsigned();
        $table->foreign('plan_id')->references('id')->on('subscription_plans');
        $table->bigInteger('transaction_id')->unsigned();
        $table->foreign('transaction_id')->references('id')->on('transactions');
        $table->timestamp('subscribed_at');
        $table->timestamp('expires_at')->nullable();
        $table->boolean('is_active')->default(true);
        $table->json('benefits');
        $table->timestamps();
    });
}

and the user model (User.php) relations and methods connected to subscriptions as below

// user - subscription relation 
public function subscriptions(): HasMany
{
    return $this->hasMany(Subscription::class, 'user_id')->orderBy('subscribed_at', 'asc');
}
.
.
.
public function createSubscription(Plan $plan): Subscription
{   
    $transaction = $this->createTransaction($plan);
    $data = [
        'plan_id' => $plan->id,
        'transaction_id' => $transaction->id,
        'is_active' => true,
        'subscribed_at' => now(),
        'expires_at' => now()->addDays($plan->validity),
        'is_active' => true,
        'benefits' => $plan->benefits
    ];
    $this->subscriptions()->update(['is_active' => false]);
    return $this->subscriptions()->create($data);
}

on this update : $this->subscriptions()->update(['is_active' => false]); the subscribed_at column also get affected with current timestamp.
And the table column after migration looks as below.

But after changing the subscribed_at as nullable in migration it's working as expected. Still, have a question why is the column being affected on every update?
Did I miss anything?