Normally, Bitcoin mining has fairly high variance - you only get paid when you find a block, and because blocks are relatively rare and found at random intervals your income varies a lot too. MtRed were what's known as a PPS pool, which meant they basically ate the variance and paid miners based on the amount of mining work they did.

The trouble is that running a PPS pool requires a huge buffer of your own funds to cover payouts during periods of bad luck, and if the buffer isn't big enough the pool will inevitably go bankrupt. It appears this has now happened to MtRed and it sounds like they only have enough funds to pay out half the money they already owe miners. (Technically, over a long enough period of time PPS pools will go bankrupt with 100% certainty no matter how big the pool buffer.)