why does dills explanation of jems state of dress almost land him in trouble

Dill makes up a story about playing strip poker to explain Jem's missing pants, and Jem says they were playing with matches rather than cards, as playing cards would be considered unforgivable by his father.

I admired my brother. Matches were dangerous, but cards were fatal.

"Jem, Scout," said Atticus, "I don’t want to hear of poker in any form again. Go by Dill’s and get your pants, Jem. Settle it yourselves."