Basically the Preah Vihear temple has been a point of contention between Thailand and Cambodia since before most of us were born. A Cambodian soldier was killed recently so it sparked more recognition to the topic.

Former Prime Minister (essentially still the leader in power) Hun Sen leaked a phonecall audio of Thai PM Paetongtarn Shinawatra, where she was treating him with more respect and talking bad about the Thai military. This is concerning because typically citizens don't want their leader to be more than neutral with foreign entities, plus see: Thai history with military coups. The Hun and Shinawatra family used to have a strong alliance so this is Trump-Elon level of newsworthy. Thai citizens protested her place in leadership so it also flamed anti-Cambodian sentiments further.

In Cambodia recent news, the government amended the constitution where it says Cambodian citizenship cannot be stripped. Now, it can be revoked. So people are worried that it can affect political dissidents, activists, and regular people and take away their human rights. THEN, military conscription became mandatory in over 20 years (I think?) for young adult males. So desertion could potentially be considered as treachery.

As of now, both countries are evacuating their citizens from border cities because of what has happened. Of course, it does not touch their main capitals of Bangkok and Phnom Penh (yet), so people worry this might be a full scale war. And then there is that alleged Hun Sen map leak.