This is not unusual, but it detracts from the look of the plant and is a sign that attention is needed.
The simplest approach is to dig up the whole plant in spring, or late summer, improve the spoil and then replant with those bare woody stems below soil level. They will produce roots and a large new plant will be the result.
Better, is to divide the plant into pieces – each with both leaves and roots. These can be replanted in the same site, or elsewhere.
Finally, you can also snip off individual side crowns and either replant them straight away – most should root – or grow them on in pots for few months in a sheltered place.