Whenever Trainer Cliff has me lift heavy weights, we always do lots of warming up first. Lots! Then warm up sets and then finally the actual heavy lifting, which is typically not very many lifts. Warm-up sets of 8 reps, then more weight added, then a set of 6, and so on. Before I get to my new high lift, I am down to 1 rep.
Maybe you are instinctively doing the same?