Both husband and wife travel quite a bit and have always received TSA Pre-Check on both boarding passes until just recently (August 2018). We always book together yet this time one boarding pass was Pre-Check and the other was not????? What has changed?
There is no concept of a "spouse" or even travel partner when it comes to PreCheck - all travelers are treated as individuals regardless of their relationship or how the tickets were purchased.
Thus the real question you're asking is why did one passenger not get PreCheck.
PreCheck always includes a random component. Even if you have paid to obtain it, there's always a chance that you will not be given it on a specific flight - which may be what has occurred here. If you haven't paid to obtain PreCheck status then the random factor plays a much later role, to the extent that most travelers who haven't paid (or don't have Global Entry/etc) will normally not receive PreCheck.
It's also possible that your relevant details were not entered correctly on the ticket. This could include the known traveler number/PreCheck number, or even details like name/date of birth, which can trigger PreCheck not being given if they don't match the travelers correct details.
Doc's answer is absolutely correct but is mostly from the perspective that both passengers are TSA PreCheck members. I'm giving a different perspective if only one traveler (you) are a member.
TSA PreCheck isn't determined strictly by TSA, and non-PreCheck members can get it, if the airline so desires. Though according to American Airlines at least, the TSA as of late has been reducing this ability.
Anecdotally, every time I've booked multiple people under the same reservation as me (a CBP Global Entry/TSA PreCheck member) on Frontier Air, everyone else in my party was given it, even adult non-enrollees. This worked for me even on a domestic flight as of 3 days ago (December 8th, 2018).
TSA PreCheck is a partnership between the airline, the airport, and the TSA. Each airline has separate rules for who can get the check and not, and TSA appears to be re-negotiating these agreements with several airlines.
If you have assumed that your spouse was getting PreCheck because you had it, thank the airline that gave it to you, as they don't have to, and don't/can't always.