This is what the rate is, period. No explanations are required. If you continue to be busy, move to the next level by increasing rates on new matters for existing clients who are not your bread and butter. You are required to inform existing clients of any fee increases, and the appropriate way is to amend your engagement agreement. A short note is all that s required and is probably the best way to accomplish this.
Long explanations imply uncertainty and apology, when no apology is needed.
You can also strengthen your position by using technology to lower costs and increase profits before raising your rates. Finally, when you re confident about these preliminary forays into higher fees, roll out your new fee structure to all remaining clients. Always talk personally with major clients in advance of any formal notice of a fee raise. Confirm with clients that a fee increase still fits within their budget for legal services. Doing this highlights your sensitivity to their economics and takes their opinions into account.