From the Wikipedia page on Heworth 
On 24 August 1454, a skirmish took place and was the first meeting of the two families involved in the Percy-Neville feud, the feud which eventually helped provoke the Wars of the Roses. Historical analysts have described an attack on the Neville family's wedding party by Lord Egremont; numerous contemporaries regard it as the very first military action of the Wars of the Roses.
The Neville family was returning to Sheriff Hutton castle following a wedding between Sir Thomas Neville and Maud Stanhope. Stanhope was the heiress and niece of Baron Ralph de Cromwell. Cromwell had previously confiscated Percy strongholds such as Wressle and Bunwell after Henry 'Hotspur' Percy's death in 1403; the thought of those properties one day being handed over to the Neville family angered Lord Egremont greatly
Egremont decided to ambush the Neville family's returning wedding party at Heworth Moor, along with 1,000 retainers from York. The Neville family were said to have given a good account of themselves and defended themselves well in the skirmish.