Unfortunately, lwjgl applets on macs have been nothing but trouble for me. The problem has to do with the Java version that Apple provides, it is not 100% compatible with the official version. I had experienced many challenges including applets not able to re-load, applets stays on top even when switching to a different tab, etc.
Apple has recently updated their Java, so try a system update the see if that solves the problem. Personally, I think it may be good that Sun (Oracle) will be maintaining the Mac JVM now, assuming that they do a good job.