Why the button anarchy when 2 line text in android?

I am develop in Android , I trying to set the UI of button by xml.

The btn_corner.xml(put in drawable) is like the following: This xml is use to set button UI.

<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://ift.tt/nIICcg" >
    <item>
        <layer-list xmlns:android="http://ift.tt/nIICcg">

            <item>
                <shape android:shape="rectangle">
                    <solid android:color="#FFFFFF" />
                    <corners android:radius="5dp" />
                    <stroke android:width="1dp" android:color="#B1B1B1"/>
                </shape>
            </item>

            <item android:bottom="30dp" android:left="3.5dp" android:right="3.5dp" android:top="1.5dp" android:drawable="@drawable/btn_shadow">
                <shape android:shape="rectangle">
                    <solid android:color="#F2F1F1" />
                    <corners android:radius="5dp" />
                    <stroke android:width="1dp" android:color="#FFFFFF"/>
                </shape>
            </item>


        </layer-list>
    </item>

</selector>

And the part of xml ,

<LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_alignParentTop="true"
        android:orientation="vertical" >

        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            >

            <Button
                android:id="@+id/findCameraButton"
                android:layout_width="0dp"
                android:layout_height="60dp"
                android:layout_weight="4"
                android:textColor="#00BBFF"
                android:background="@drawable/btn_corners"
                android:text="@string/label_find_camera" />

            <Space
                android:layout_width="0dp"
                android:layout_height="match_parent"
                android:layout_weight="0.5"
            />

            <Button
                android:id="@+id/cameraRecordButton"
                android:layout_width="0dp"
                android:layout_height="60dp"
                android:layout_weight="4"
                android:layout_gravity="top"
                android:textColor="#00BBFF"
                android:background="@drawable/btn_corners"
                android:text="@string/label_camera_record" />

            <Space
                android:layout_width="0dp"
                android:layout_height="match_parent"
                android:layout_weight="0.5"
            />

            <Button
                android:id="@+id/cameraSnapshotButton"
                android:layout_width="0dp"
                android:layout_height="60dp"
                android:layout_weight="4"
                android:textColor="#00BBFF"
                android:background="@drawable/btn_corners"
                android:text="@string/label_camera_snapshot" />
        </LinearLayout>

But it show like the following:

Why the Button will move down after the button text change to 2 line?

Thanks in advance.