Why did Ali postponed swearing loyalty to Abu Bakr?

Shia and Sunni disagreed on some important things. Shia thinks that Muhamad select Ali as a caliph. Sunni thinks no. In fact, an answer in this stackexchange, says that Muhammad actually primed Abu Bakr as next caliph. It seems that they agree on one thing. Ali didn't agree to support Abu Bakr from the beginning. It took a few months before that to happens. What happened? The Shia version is that Ali feels that Abu Bakr is an usurper. What's the Sunni version?