FOMO plays a part, however the price increase takes some time as the miners begins to adjust to this new reality of getting less bitcoin with each block they mine, so miners are the ones which set a floor as they are not willing to sell their coins for a loss, and this floor keeps getting higher as they need to remain profitable to keep mining bitcoin, eventually traders and investors catch up and this is what causes the slow growth we see after the halving.
Of course FOMO plays role in the process of rising bitcoin prices, but in any case a correction can always be expected. Some miners should also consider selling their bitcoin even though the amount of the reward has been reduced, this is done to cover operational costs even though the price is low. I don't think that's the case, but they have to keep living amidst the increasing difficulties.