Couple of points: the A & E system is widely abused in all hospitals - not simply Wales'. No one's quite sure why this is, but some think it's because GPs have created systems which prevent patients seeing them quickly and easily, while others believe the media are culpable (no surprise there, then) in generating hysterical reactions through stories regarding 'flu, long waiting times and other intangible threats to health. There are also a regular number who overdose. Suicide and attempted suicide rates have rocketed from what used to be a pretty high level of 800 per week across the UK to 5000 per week. That adds a significant number to the load. Finally, we're an ageing population and when the very old, who are often doubly incontinent, present any sort of problem to the care homes, they're shipped off to A & E. The Hospital, therefore, becomes the last stop, and the ageing are an increasing issue.
The second point is that the Welsh Ambulance system is broken. Expect to wait an hour for a 999 responder on a weekday if you have a heart attack, and their ability to ferry patients between hospitals is severely compromised through poor planning and decision making.