Why does my eyelid get stuck?

It can be associated with blepharospasm.  There are also a variety of other conditions that can be associated with apraxia of eyelid opening.  The list has a lot of rare entities on it.  It is reasonable to consider being assessed by a neuro-ophthalomologist for these symptoms if only to be reassured that your issue is not caused by one of the scarier causes of apraxia of eyelid opening.  No this is not related to conjunctival irritation or pollen.

Would need to physically examined to determine but irritation of the conjunctiva can cause the eyelid to get stuck particularly in these periods of high pollen. Consider using eyedrops to periodically rinse out the conjunctiva.