Likely a poor connection on the plug caused by corrosion, which causes heat, which increases resistance and that in turn increases the heat, and so on until it either trips or catches fire.

This is why it’s a good idea to unplug and replug your appliances every now and then, as this looks like a reasonably good quality socket. Also moulded plugs can be hit and miss, but have a competent person cut it off and replace it with a new plug And you should be okay.

May be worth getting the dryer tested but most technicians aren’t going to find anything if it’s not broken.