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

‘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. Even in dry seasons it is a danger to cross it, but after these autumn rains it is an awful place."

The autumns rains create a mud that acts in the same way as quick sand. At that time of the year, a man would be sucked in with no chance of escape.