Yup, experienced this at EDC 2017. Rates went from ~$120 per night before the dates were announced, to over $300.
Only way I've found to beat the price hikes is, if you know roughly when a festival will be, put in reservations for multiple weekends before the actual dates are announced. Then, after you know which reservation to keep, you cancel the other ones.
Example: EDC happens in early- to mid- May. So book reservations for every weekend in from late April to late May, then cancel the others once the dates are announced.
He was not master of his domain...