High network volume

If there is a high volume of transactions going through the Bitcoin network, there might not be enough space available in each new block to include your transaction. This can ultimately delay your transaction.

Fee spikes / sending with insufficient fees

On the same note as the previous line, if this is high network traffic, fees will typically rise. If fees spike significantly and your transaction is broadcast with a much lower fee than the average, then miners will typically favor transactions with higher fees first.

Your transaction will remain pending until the fees come down, or until your transaction leaves the mempool and is canceled after around ~2 weeks. If your transaction is automatically canceled, the funds will return to your account.

Insufficient network confirmations

Bitcoin transactions require a specific number of network confirmations before they are completed. If your transaction hasn't reached the required amount of confirmations, it will remain as Pending.