Why are Thailand and Cambodia fighting at the border?

BBC:

Simmering tensions between Thailand and Cambodia have once again exploded along their shared border, derailing a fragile ceasefire backed by US President Donald Trump.

At least three Thai soldiers and seven Cambodian civilians have been killed since Monday, with the countries accusing one another of starting the violence.

The clashes, which have also seen Thailand launch airstrikes along the border, are the most serious since the ceasefire was agreed in July.