I don't know whether the question of "Why" they believed in an "imminent" return of the Lord was raised in the article, or by utilitas; the scriptural truth is:
The 12 and their Jewish believers in Christ expected Christ to return <i>any minute</i> to destroy His enemies and restore the Kingdom to Israel. They died off waiting but did receive "wisdom" (II Peter 3:16,17) from Paul that the return might be delayed indefinitely in order for more souls to be saved.
Paul's converts on the other hand believed they would be swept [without death] into the heavens [with resurrection bodies] prior to the return of Christ. Just as Christ's return to earth was held imminent, so was the catching away of Paul's believer's imminent. For believers today, the catching away (or rapture) is imminent every day.