The border was disputed in the aftermath of WWI. In 1919 there was even shooting where the Czechs drove Polish troops from the Cieszyn Silesia area. There were a series of negotiations finally resulting in a peace treaty in 1925. But the Poles still felt dissatisfied with the border. Combined with other things it poisoned the Czech-Polish relationship during inter-war years.

So come 1938 and the Germans were clearly preparing to seize the Sudetenland. This both alarmed and interested the Poles. Certain areas were considered strategically important. And while they weren't happy with the Czechs having certain rail hubs, namely the town of Bohumín, it was outright alarming to think of the Germans having them. So the Poles moved in to occupy Bohumín. While they were at it they also grabbed a bunch of other border areas, because once you are invading another country, you might as well grab all the disputed territory.

All in all it was an opportunistic move to overturn a treaty that the Poles weren't happy with. Czechoslovakia was getting carved up and the Poles weren't to going to let such a moment go to waste. Unfortunately for them it helped legitimize the German annexation, and undermined French and Czech attempts to create an anti-Nazi alliance in the East.