Recent trend is win toss, field, chase small target and win the match! I believe reason might be inability to score big while batting first.
This IPL was unlike other IPLs teams preferred to bat first after winning the toss…and they had considerable success!.
However as tournament entered the last phase (post 40th match) teams decided otherwise, they opted to chase if they won the toss.
Here is brief gfx of matchwise scores.