I thought it was those pesky Poles refusing to provide a German land corridor to enable intra-territorial transit between Germany and Germany’s exclave East Prussia. That and ethnic Germans allegedly being harassed in Poland.

First one is definitely true and isn’t emphasized much and tbh I feel like that demand wasn’t unreasonable. Shipping people and things and providing defense would be a lot harder to an exclave than to contiguous territory. They did seriously overreact by invading, of course, and it seems like Mr H had some serious temperamental issues.

Second one I’ve never researched enough to know if it’s true or German propaganda.

I don’t recall the Germans ever claiming that Poland was about to invade them? Maybe I missed it.