Let's say that in market XYZ the bid-ask spread is 150-100. If I place a limit order at 100 maybe I will not be filled so fast, because there are already some orders queued up. Therefore I place it at 101 to be in front of other people. Another guy sees my order at 101 and places his order at 102 to be filled before me. Compound this behavior for all market participants. More participants = more liquidity = tighter spread.
This is of course a very simplistic explanation but that's the basic principle.