Because of the smell and cloudiness coming and going, I'd say this wasn't a bacterial bloom but rather a case of something inside the LR, a big worm or a boring clam, dieing and polluting the water. It could have been just barely hanging on and finally kicked the bucket or it could have had other dead stuff in its hole sealing it off from the main water. The "seal" released and all the decomposing crud went into the water column.
Can't be sure without looking at the LR, but this is my best guess.