Why did a false step in the mire mean death at that time of the year?

A false step in the mire would cause a person (or animal) to be sucked into the bog..... similar to quicksand.

“That is the great Grimpen Mire,” said he. “A false step yonder means death to man or beast. Only yesterday I saw one of the moor ponies wander into it. He never came out. I saw his head for quite a long time craning out of the bog-hole, but it sucked him down at last.