Heroku - why would I get Error R12 (Exit timeout) when pushing a release to Heroku?

Question

Occasionally, whenever I push a release to Heroku shortly after I would receive the following error (I'm running 2 512MB dynos):

2014-11-21 00:38:30.216
188 <45>1 2014-11-21T00:38:29.163459+00:00 heroku web.2 - - Error R12 (Exit timeout) -> At least one process failed to exit within 10 seconds of SIGTERM

I'm using the unicorn app server, unfortunately only 1 per unicorn worker per 512MB dyno (since at its peak, my app RSS is 320MB - yea, go figure, some bloat is happening). Not sure if this helps, but I'm on Cedar14 with Preboot enabled. UNICORN_WORKERS is set to 1.

Here's my unicorn setup. Should I be concerned with this error?

And while we are on this topic, is db pool size 15 too large for my 2 dynos (I'm using Postgres standard which allows up to 120 concurrent connections).

worker_processes Integer(ENV['UNICORN_WORKERS'] || 2)

timeout Integer(ENV['UNICORN_TIMEOUT'] || 25)

preload_app true

before_fork do |server, worker|
  Signal.trap 'TERM' do
    puts 'Unicorn master intercepting TERM and sending myself QUIT instead'
    Process.kill 'QUIT', Process.pid
  end

  if defined?(ActiveRecord::Base)
    ActiveRecord::Base.connection.disconnect!
  end
end

after_fork do |server, worker|
  Signal.trap 'TERM' do
    puts 'Unicorn worker intercepting TERM and doing nothing. Wait for master to send QUIT'
  end

  # other settings
  if defined?(ActiveRecord::Base)
    config = ActiveRecord::Base.configurations[Rails.env] || Rails.application.config.database_configuration[Rails.env]
    config['reaping_frequency'] = Integer(ENV['DB_REAPING_FREQUENCY'] || 10)
    config['pool'] = ENV['DB_POOL'] || 15
    ActiveRecord::Base.establish_connection(config)
  end

end