I believe Monarch ordered the 30 737MAX in 2014 with delieries starting in 2018 and that it went into the Boeing order book with Monarch as the Customer. They took options for a further 15 and it is possible that the confirmation date for those options is coming up (though it seems a bit early if first deliveries are say in 2020.)
The only explanation I can see for making an announcement now would be if they are putting in place a means to finance these aircraft such that the cost to Monarch before delivery will be minimal thus not a drain on their cash-flow.