Cucumber is firm when first starting to grow but it then turns soft and mushy at the end of the cucumber.
Cucumbers Firm Then Turns Soft
Certified GKH Gardening Expert
This could be Belly Rot or another fungus called Angular Leaf Spot.
Crop rotation is important to correct this issue in the future. Damp weather conditions can be a cause with lack of air circulation, sometimes we can't control all these things as gardeners.
You can try to salvage the cucumbers by picking them smaller then you normally would.
Here is more information.