To reproduce:

LineSeries lineSeries = new LineSeries();
lineSeries.DataPoints.Add(new CategoricalDataPoint(20, "Jan"));
lineSeries.DataPoints.Add(new CategoricalDataPoint(22, "Apr"));
lineSeries.DataPoints.Add(new CategoricalDataPoint(12, "Jul"));
lineSeries.DataPoints.Add(new CategoricalDataPoint(19, "Oct"));
lineSeries.LegendTitle = "Series A asdafsafasfd fgsdfg sdfs dfg dafsvsd";
this.radChartView1.Series.Add(lineSeries);
LineSeries lineSeries2 = new LineSeries();
lineSeries2.DataPoints.Add(new CategoricalDataPoint(18, "Jan"));
lineSeries2.DataPoints.Add(new CategoricalDataPoint(15, "Apr"));
lineSeries2.DataPoints.Add(new CategoricalDataPoint(17, "Jul"));
lineSeries2.DataPoints.Add(new CategoricalDataPoint(22, "Oct"));
lineSeries2.LegendTitle = "Series B sdf asdf adfasdfas dfadsf asdf sadf awef";
this.radChartView1.Series.Add(lineSeries2);
LineSeries lineSeries3 = new LineSeries();
lineSeries3.DataPoints.Add(new CategoricalDataPoint(13, "Jan"));
lineSeries3.DataPoints.Add(new CategoricalDataPoint(14, "Apr"));
lineSeries3.DataPoints.Add(new CategoricalDataPoint(12, "Jul"));
lineSeries3.DataPoints.Add(new CategoricalDataPoint(25, "Oct"));
lineSeries3.LegendTitle = "Series C sdf asdf adfasdfas dfadsf asdf sadf awef";
this.radChartView1.Series.Add(lineSeries3);

this.radChartView1.ShowLegend = true;
this.radChartView1.ChartElement.LegendPosition = Telerik.WinControls.UI.LegendPosition.Bottom;
radChartView1.ChartElement.LegendElement.StackElement.Orientation = Orientation.Horizontal;

Workaround: specify minimum height for the legend

radChartView1.ChartElement.LegendElement.MinSize = new Size(0, 50);