I have to claim ignorance at this point, so I have to ask the questions. I did not know this could happen for I use coinbase and there has never been much of an issue for my transactions.
Coinbase pays the fee for you. If the fees continue to rise, then I suspect that eventually Coinbase will no longer be able to afford to do that, and they will eventually start subtracting the fees that they pay from your balance.
So when there is still a backup of unconfirmed transactions like this, why do some transactions go through and some do not?
It is determined by the amount of fees per byte of transaction size that you pay. The miner (or mining pool) that solves a block gets all the transaction fees of all the transactions that get their first confirmation in his block. The miner (or mining pool) is allowed to choose which unconfirmed transactions they want to confirm in their block. The miner (or mining pool) is not allowed to create blocks that are larger than 1 megabyte in size. Therefore, there is an incentive for the miner (or mining pool) to choose the transactions that pay the highest fee per byte until their block is full. Then all the remaining transactions have to wait in line until the next block. If additional transaction are broadcast while everyone is waiting for the next block, and those additional transaction pay a higher fee, then the additional transactions will get confirmed, while the existing transactions that pay a lower fee will continue to wait.
If they are not confirmed for a long period, will they go back to the sender,
Effectively ,yes. Literally, no.
In a literal, technical, way the bitcoins don't ever leave the control of your wallet until the transaction is confirmed. Therefore, if the transaction is unconfirmed then it is technically possible to replace that transaction with a different transaction, and in doing so, the original transaction becomes invalid and vanishes from the bitcoin network.
Effectively this means that until the transaction is confirmed, your wallet software can choose to forget that it sent the original transaction and show those bitcoins as being "back under the control of the sender".
chancing the receiver loosing funds on something they already gave out?
Yes. This is why the receiver of the bitcoins should never give out anything of value in exchange for bitcoins unless either the transaction has enough confirmations to reach the risk tollerance of the transaction recipient, or the transaction recipient has a trust relationship with the sender.