suppose for the sake of argument that each individual miner has the same amount of hashing power of one hash per unit of time (a "tick").
the probability that a miner solves their block in the next tick increases with the number of hashes already checked, since the nonce space is finite.
colluding miners could agree to pay the coinbase transaction to the same address and thus work on solving exactly the same block. they could divide the nonce space amongst them and thus exhaust the nonce space faster than solo miners.
by agreeing to redistribute the rewards of their mining amongst them, the colluding miners have an expected payout for the next tick that increases faster than that of solo miners, since they are exhausting their nonce space faster.
so isn't it rational for miners to collude? could this create a tendency to monopoly in mining?