why those southern black baptist preachers start doing that singing-type preaching?

That's the "Gospel" style of worship that injects emotion into the service that you don't find in other denominations. I believe the style grew out of the spiritual songs sung during the slavery era, which gave rise to rock n' roll too of course. Hopefully you've had a chance to experience these services, they shed a whole new light on worship.

These practices reflect a mixture of native African religion with Methodist and Baptist beliefs. The "call and respond", the "amen corner", the "gettn' happy", the "ring and shout", and speaking in tongues, are all rooted in an altered state of consciousness (ASC) found in most old tribe-based religions world-wide.

Sorry ...I Iive in the south..and it's just not black baptist who preach like that...I've seen white baptist use the same preaching style...

Sometimes it's a segue into the music that will come after a certain part of the sermon.

I've always thought happy praying is better than the morose ones.

Mental Illness.

Because they know it makes people feel all gooey and fussy and they give more money when they give from their emotions rather than from their common sence.

It is in their soul.