Allium bulbs splitting

I grow a number of winter/spring growing Allium species in a cold greenhouse. Last weekend I was looking through the pots to see how they were doing and while all had survived; some had thrived and for others the one or two bulbs, I'd started with, had split into two to four small bulbs inside the "skin" of the original bulb.
Does anyone know what causes this? What can I change in the care regime to encourage bigger bulbs?

 

I think the weird weather this year is doing strange things to lots of plants, in the spring it was very stop start regarding growing, summer isn't turning out much different.

I'm finding it one of the latest summers of all time regarding flowering of most plants.

 

Aehm... Why would you want to keep them from splitting? This is one of the ways the are multiplying and rejuvenating themselves.

So dig them up every 3 years or so, break the bulbs apart, and gift them to neighbors, friends or family unless you have the place for planting them of course.

I do the same with almost every bulb-based plant. Garlic is one of the most prominent. Plant one clove and harvest a whole bulb 8 months later.

 

@Black Dog Full agree regarding garlic and shallots, less keen on my main crop onions splitting though.
Ornamental onions like Mars will split into smaller bulbs if not planted deep enough resulting in a cluster of leaves and no flowers the following spring. Personally I find Mars is a thug and I am striving to get rid of it.
The alliums I am having the issue with are small alpine species that like well drained growing conditions and dry resting conditions in summer. As these tend to be small plants with delicate flowers, growing in pots under cover enables you to see the flowers easily.
Many of the bulbs are about 1 cm in diameter if that splits into 4 miniature bulbs, then you end up with just leaves and if they split again that's probably good bye.
These are the sort of thing I'm talking about.
Allium litvinovii

Allium fimbriatum purdyii

Allium hyalinum


Allium bolanderi

 

Had a suggestion from someone I talk to occasionally.
Which was I need to keep them growing for longer.
Extending the growing season for them does make sense to me.

 

@JWK
As I grow them in plastic pots, in an unheated greenhouse there is a chance that they the compost is getting too hot and causing them to shut down for the summer too soon. So move them out of the greenhouse to a cooler, still sheltered from the rain, spot. Or a sand plunge where the pots are buried in a bed of sand, this should moderate conditions and even out water supply.

 

Just spoken to a mate who grows a lot of alpines, amongst other things.

He suggested it could be an intermittent watering problem.

 

@pete your mate could well be right, over the years I have lost plants to overwatering so am on the cautious side.