On Wikipedia, it is stated that:
Earlier, Hitler's foreign policy worked to weaken ties between Poland and France, and attempted to manoeuvre Poland into the Anti-Comintern Pact, forming a cooperative front against the Soviet Union.
Poland would be granted territory to its northeast in Ukraine and Belarus if it agreed to wage war against the Soviet Union, but the concessions the Poles were expected to make meant that their homeland would become largely dependent on Germany, functioning as little more than a client state.
The Poles feared that their independence would eventually be threatened altogether.
But I can't find the exact demands the Nazis had during this period. I am assuming that the demands were return of the City of Danzig and East-Prussian territories? This would have effectively turned Poland into a landlocked country and dependent on Germany for naval passage and transit port. (Unless the Poles succeeded in getting the Germans to carve them a corridor through Ukraine all the way to the Black Sea).
But that doesn't explain why Poland thought it would threaten their independence. There are many landlocked countries who are independent. If Poles didn't trust Germans with transit pact, they could have made a similar one with
Lithuania or Latvia (Poland tried to restore ties with Lithuania but Soviet Union had warned the Baltic States to not to get too friendly with Poland), both of whom had naval access and shared borders with Poland. Occupation of Baltic states by Soviet Union hadn't occurred at that time. Or could it be that they were concerned that without a maritime access to the Baltic sea, in event of a German invasion, French and British expeditionary forces would face troubles in landing to relieve Poland? (Although I doubt Allies would have done that, they could always open a Western front on French-German border which eventually they did).
Are the exact demands known?