I've seen it plenty of times actually over past year or so. It's a java issue I suspect. If LW has been on for a long time, virtual memory builds up making LW slow to respond. Be patient or use force quit as a very last resort. (But yes, your ShutDown immediately option may be one reason.)

How to stop build up of Virtual Memory (VM)? (a) Reduce incompletes; Tip: Purge the Downloads Queue! (click on blue link).
(b) Reduce shared files if over 2,000 or more than 1,000 video files.
(c) Share less from an external drive if used using Firewire 400, if using a FW external drive.
(d) Close LW periodicaly which resets VM.
(e) Under preferences settings, Advanced > Performance, disable Ultrapeer capabilites, as that can slow LW down whilst the peers attempt to find other UP's.