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One of my favorite perennials, Lupine! Lucky you!

https://www.gardeningknowhow.com/ornamental/flowers/lupine/growing-lupines.htm

Lupine is a fussy plant and if the soil isn't just to its liking, it will languish. It does not like clay, akaline or poorly-draining soil but doesn't like to be dry either. Here is an article that goes over lupine's particular needs, and problems growing them. Pat particular attention to chlorosis. http://homeguides.sfgate.com/lupine-troubleshooting-66404.html

That is a hybrid lupine. To save seeds, wait until the pods turn tan colored. They won't be ripe seeds if you pick them now. Carefully pick the pod; when fully ripe they "explode" flinging seed several feet away. Remove seeds from pod and allow to dry out of direct sunlight for several days. Store in a cool, dry place. There are 2 things that must be done to get the seeds to germinate. 1. Place in plastic bag with damp paper towel and place in refrig' for 2 weeks. Scarify or nick the seed and soak seeds in warm water for several hours. Seeds that sink are ready to plant. Scarify floaters again and soak until they sink. Lupine has a taproot and is best sown directly in the garden but only about 1/4." deep. Keep soil moist and be patient. Seed may take 2 - 12 weeks to germinate. Since it is a hybrid seed, flowers may not look like the parent plant. I suggest buying a pack of seed. The seed company has already treated the seed so you won't need to do anything but plant them.

It is a pH issue. You will have to test the soil or have it tested to figure out which way it has turned. They you can correct it with dolomitic lime to raise pH, or iron sulfate, to lower it.

This article will help you to care for these plants: https://www.gardeningknowhow.com/ornamental/flowers/lupine/growing-lupines.htm

This can happen as the weather presents itself to be suited for the plant. This can trigger flowering or growth. It is not a problem and nothing will need to be done.

Depending on what country you are located in, and the climate there, this can be caused by a few things. It is no cause for concern, however.

Lupines flower during the long days, which we are waning from. As flowers start to fade and drop off leaves will start to grow again for the vegetative cycle in preparation to flower next summer.

If this was planted recently, it could have been forced to flower, then changing its environment quickly can shock it a bit forcing it to revert to a vegetative state again.

This article will help you to care for these: https://www.gardeningknowhow.com/ornamental/flowers/lupine/growing-lupines.htm