According to IOTA Engineering:
IRI 1.5.3 addresses the blowball problem seen on Mainnet.
What exactly is the blowball problem and what causes it?
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The blowball problem is this phenomena where a huge quantity of new transactions select more or less at the same time the exact same transaction (most of the time the latest milestone) as branch and trunk transaction.
On a tangle vizualizer this phenomena is visible in the form of "balls" where the center of the ball is a milestone, and the ball consist of a huge amount of tips.
It's a problem for the convergence of the tangle because when a walker reach the center of the ball it will have the choice between a wide variety of tips with more or less the same transition probability. Intuitively, we can understand that it take more time to obtain a solid consensus when facing such a wide variety of choice. And once the consensus is reached (i.e. some of the tips originally forming the ball have accumulated enough weight to attract the majority of the future walkers), all the other tips forming the ball will be "left behind" (i.e. forever tips).
The causes can be :