Creating a mountain of blocks that is not there before

I was playing one time and when a glitch happened. I went back to a place I went just a few seconds ago... But, something changed from my original veiw... The river, as you can see at the pic i have attached, is now blocked by a mountain of blocks(the encircled mountain)  I swear I didn't build that thing, and there is no mountain blocking the river before... I am very trigerred .... I don't know what and why it happened....