Since Ext 4.2.0 ComboBox seems to register a filter on store it is bound to (property 'queryFilter'). When store has remoteFilter: true, it is forced to do a remote load immediately. On 4.1.3 filter was not registered, and store never issued server request until user interacted with the combo.

Sample provided (ready to pase in try.sencha.com) shows the problem. Just view browser network activity (sample is using JsonP - look for topics-remote.php scripts)

Code:
Ext.require([    'Ext.data.*',
    'Ext.form.*'
]);
Ext.onReady(function(){
    Ext.define("Post", {
        extend: 'Ext.data.Model',
        proxy: {
            type: 'jsonp',
            url : 'http://www.sencha.com/forum/topics-remote.php',
            reader: {
                type: 'json',
                root: 'topics',
                totalProperty: 'totalCount'
            }
        },

        fields: [
            {name: 'id', mapping: 'post_id'},
            {name: 'title', mapping: 'topic_title'}
        ]
    });

    panel = Ext.create('Ext.panel.Panel', {
        renderTo: Ext.getBody(),
        title: 'Search the Ext Forums',
        width: 600,
        bodyPadding: 10,
        layout: 'anchor',

        items: [{
            xtype: 'combo',
            store: {
                pageSize: 10,
                model: 'Post',
                remoteFilter: true,
              remoteSort: true
            },
            displayField: 'title',
            anchor: '100%'
        }]
    });
});