This can happen for a large variety of reasons. Would help to know if this was something that didn't happen previously and has suddenly occurred. If it was something that was happening previously (with the same amount of instances of EQ) then perhaps looking more into the settings of what is causing it would assist. What zone you are loading into may also be relevant, as loading into guild hall or other private instances can cause issues on login because the zone isn't ready yet. Things that may be relevant are any settings you changed recently. Any plugins you are running, paying attention to the total system memory available versus the usage (using more memory than is available can certainly cause issues like that).
Without more information it's quite hard to know what could cause it.