What's the Most Common Reason Merchants Get Funds Held?

I’ve seen merchants obsess over rates when choosing a processor but more often the real problem is funding holds.

For those working with merchants, or owning businesses:

• What is the most common reason you have seen funds held up?
• Chargebacks? Sudden volume spikes? Underwriting issues? Fulfillment delays? Or what?
• Merchants could have done something else to prevent this.

I would love to hear from people who have actually used it.