The rapid growth in the parallelism of multi-core processors has opened up new opportunities and challenges for parallel simulation discrete event simulation (PDES). PDES simulators attempt to find parallelism within the pending event set to achieve speedup. Typically the pending event set is sorted to preserve the causal orders of the contained events. Sorting is a key aspect that amplifies contention for exclusive access to the shared event scheduler and events are generally scheduled to follow the time-based order of the pending events. In this work we leverage a Ladder Queue data structure to partition the pending events into groups (called buckets) arranged by adjacent and short regions of time. We assume that the pending events within any one bucket are causally independent and schedule them for execution without sorting and without consideration of their total time-based order. We use the Time Warp mechanism to recover whenever actual dependencies arise. Due to the lack of need for sorting, we further extend our pending event data structure so that it can be organized for lock-free access. Experimental results show consistent speedup for all studied configurations and simulation models. The speedups range from 1.1 to 1.49 with higher speedups occurring with higher thread counts where contention for the shared event set becomes more problematic with a conventional mutex locking mechanism.